Grammar: "Just because A, doesn't mean B"

No, that isn't correct. "Just because" in that sentence is used in the sense of "since"; in trying to understand the sense in which it's being used, it should be acceptable to discard "just". It would be closer to correct to say something such as, "Because I stopped eating, I must be full", even though clunkily back-to-front, but one should not use this sort of construction to convey the opposite. The speaker/writer is simply choosing the wrong construction to mean something like "The fact that I've stopped eating should not be taken to show/mean/imply that I'm full." "Just because" means "only because" in that construction, and it's incorrect to use a conjunction there in that way. Also, on a logical basis, of course it can't be literally true that any person who has stopped eating can be assumed to be full.

(ETA: Note that describing something as an "decoding idiom" or suchlike terms, as in a linked paper from another answer, does not support an argument that the construction fits existing grammar rules. There's not much room for argument that this is proper grammar, though it may be so common that we should accept it on that basis.)

I don't think that's the end of the inquiry, though. This sort of construction is used all the time by real people, and might form part of believable dialogue for many types of characters: a child, adult without much language-arts education in English, a smart person proud of working-class roots, etc. You know the people I mean — the same ones who might say "I could care less" without agonizing over whether they're being sarcastic. If it feels right, it might be right in context. Real people can be expected to use incorrect grammar in a wide range of situations, and acknowledging that might sometimes be to the aid of realism and character development. I'm reminded of Michelle Pfeiffer's Frankie yelling over a bathroom stall at Al Pacino as Johnny, "Fuck you, how I talk!"

I agree with the scond part of Iucounu's answer: Whether it's valid logic and/or grammar or not is pretty much irrelevant if it's in dialog. Lots of people use the construction. Indeed, I'd say that in many cases dialog must use bad grammar to be convincing. Like, technically if someone asks who is there the grammatically correct reply is "It is I." But almost no one says that. People say, "It's me." If the character is a meticulous English teacher, perhaps she should say, "It is I." ANy other character should say, "It's me."

All that said, I don't see anything wrong with the construction. I must disagree with the first part of Iucounu's answer. THe word "just" is not superfluous here: at the least it adds emphasis. I suppose in a sense you could say that every emphasis word is superfluous in the sense that the sentence would be meaningful without it. Like suppose I wrote, "A very old man walked agonizingly slowly across the scorching hot field." Sure, I could drop all the intensifiers and just write, "An old man walked slowly across the hot field." Or even, "A man walked across the field." The essence of the sentence is the same. But dropping the modifiers and intensifiers not only drains the color from the sentence but potentially changes the meaning.

Some say that you should not begin a sentence with a conjunction. I categorically reject that rule. Sure, a conjuction that comes out of nowhere, i.e. that does not connect the sentence to anything previously said, makes no sense. But there are plenty of times when it is perfectly logical to being a sentence with a conjunction. (Like that sentence.) And the alternative may be to either omit the conjunction, thus losing the connection, or to make an excessively long sentence just so you can "legally" include it. (See, I slipped in another one.)

The logical equivalent of "Just because A, doesn't mean B” would be You should not infer B from A. I will leave it to the pedants to argue whether A does not imply B is a valid alternative.

Were you to say this in everyday conversation with the archetypal person-in-the-street, your (grammatically correct) statement would attract more negative reactions (as in "Oooh, hark at Mr La-de-da") than "Just because ..."

As others have noted, there are often grounds for breaking the strictures of grammar for deliberate effect.

This construction has been explored over on English Language & Usage, and there's an interesting and more thorough treatment here.

The bottom line is there's nothing "grammatically incorrect" about "Just because X doesn't mean Y", but there are definitely some peculiarities. Not least that we're happier with "mean", rather than alternatives such as "imply", "prove", "establish", "reflect", etc., which (grammatically speaking) are no different.

Having said that, this is more a question of style than grammar. But in that context it's worth noting that ELU has had at least three questions about the usage, so obviously people feel it's at least "slightly odd".

I think it's generally perceived as an idiomatic usage more appropriate to informal speech than formal writing. I personally don't think there's any real justification for that perception — but since it exists, the careful writer should take it into account. In short...

Just because the grammar is correct, doesn't mean people won't think it's "not quite right." If you don't want to distract your reader with pedantic side-issues, just choose a different form of words.

From a grammar standpoint, this is fairly straightforward.

The word "because" is a subordinating conjunction — words of this type are used to create dependent clauses (also called subordinate clauses), and that's what the phrase "Just because A" is. Clauses like this attach to the main part of the sentence — the independent clause. Wikipedia on dependent clauses(1):

In linguistics, a dependent clause (sometimes called a subordinate clause) is a clause that augments an independent clause with additional information, but which cannot stand alone as a sentence. Dependent clauses either modify the independent clause of a sentence or serve as a component of it.

This is quite clear — the dependent clause has some relation to the independent clause of the sentence. The role of independent clause in our case is taken by "doesn't mean B", but it lacks something. This can be described by grammar (although of course grammar itself is not the reason it's incorrect, it's just a way of describing how we in general use language to make sense). I quote Wikipedia(2):

An independent clause (or main clause) is a clause that can stand by itself, also known as a simple sentence. An independent clause contains a subject and a predicate; it makes sense by itself.

Clearly, our purportedly independent clause is not very independent — it cannot stand by itself, because it does not have a subject. We have a dependent clause, but no independent clause, and that's not grammatical English.

(1) en.wikipedia.org/wiki/Dependent_clause
(2) en.wikipedia.org/wiki/Independent_clause

Such a sentence is grammatically incorrect. Every clause needs a predicate and a subject that executes said predicate. The clause "Just because X doesn't mean Y" has a predicate ("doesn't mean Y") but it does not have a subject executing that predicate. There is no subject, therefore the sentence is incomplete. The sentence must be formulated in a way that there is a subject. For instance: "The fact that X doesn't imply Y".

I don't know what people here are smoking... "just because A doesn't mean B" is widespread enough in colloquial usage that it may seem valid, and its meaning is clear, but it is not grammatically well-formed.

It sounds awkward because a dependent clause ("just because A") can't normally take the position of a subject. Consider how strange that sounds in the following sentence:

Just because you get here on time everyday makes me want to promote you.

The grammatically well formed equivalent is "the [mere] fact that", because "fact" is a noun that can take the place of the subject:

The mere fact that you get here on time everyday makes me want to promote you.

The fact that I have stopped eating doesn't mean I'm full.

The mere fact that you have obtained a passport doesn't mean you can go to China. (Because you also need to get a visa)

However, the advantage of "just because A" is not only that it is shorter, but also that it primes the listener to anticipate a "doesn't mean B" later in the sentence, helping them grasp the meaning more quickly. I think this is the primary reason the construction is so widespread. It is harder to predict what will follow "the mere fact that". For example:

The mere fact that she was Chinese had a dramatic impact on people's perception of her intelligence.

"Just because A" has this advantage more in written than spoken communication, because there's a certain tone of voice one can use when saying "the mere fact that you have obtained a passport" that primes the listener in the same way.