"Hence" and "hence why"

It appears that this construct used to be a lot more popular in the 1800s and continues to be used today. Going by Ngrams:

Excerpts: Barbara Heyman, 2012:

I looked at it later in 1922 with the idea of arranging it for violin, cello, and piano. Hence why we still call it the “Trio”.

Thomas A. Blackson, 2011:

The details of his pursuit were so undefined it was difficult to understand how the pursuit was supposed to work and hence why anyone should value it so highly.

Marcel Fafchamps, 2004

This may explain why the threat of stigmatization is largely ineffective against so-called informal sector firms which, as a rule, are not registered — and hence why transactions among informal sector firms remain quite unsophisticated.

In other words, this usage appears to be quite valid.

When you contrast coleopterist's Ngrams research with the answers shared between every other Google result, it appears that "hence why" is a case of "incorrect, but nobody cares" similar to "quadrilogy" (which should use the Greek "tetra-" with the Greek "-logia" to form "tetralogy" but people have been getting it wrong since at least 1865 according to the OED).

Perhaps I haven't read enough older fiction, but my intuition has always agreed with the view that "hence why" is incorrect because it translates to "[that is the reason that] why".

However, my intuition did also lead me into conflict with a more pedantic friend over whether "hence why" is ever valid on technical grounds.

The use my intuition keeps claiming validity for is that "hence why X" makes sense as a short form of "hence my decision to X" (in contrast to "hence X", rendering the decision the primary subject of the clause, rather than the action taken as a result).

The last two examples could have a comma between the hence and the why. The hence in both case means therefore and it is like saying therefore, why would anyone value. The why in both cases is the beginning of the next phrase and incidental. The first example is indeed a double usage. I wonder if these quotes were found by computer search?

After reading the various replies my mind is still inconclusive, however, the above answer: "incorrect, but nobody cares" seems to fit best.

I have always thought of 'hence' to be a word used in written communications in order to be succinct and formal, so when I hear people using the term 'hence why' in casual spoken conversation it really irritates me for some reason. It is probably due to the fact that the people saying 'hence why' are usually not very well educated people trying to sound important or clever.

Lately, I have even heard journalists and politicians saying 'hence why'. It appears they are using it as a synonymn for 'so' or 'therefore'. I quote this recent example from a political journalist:

"...both parties are free to set their own policies and priorities, hence why this story has been running in the past few days."

Surely the above sentence does not make sense? However, the folowing sentence would make sense:

"...both parties are free to set their own policies and priorities, therefore this story has been running in the past few days."

Although, adding 'why' to 'therefore' does not make sense:

"...both parties are free to set their own policies and priorities, therefore why this story has been running in the past few days."

Alas dear fellows, it is quite infuriating, hence now I must go hence in bitterness, in debt to your mercy.