Cashews are excellent sources of magnesium?

A mistake I’ve heard many English learners use happens to be really hard to explain, so I’ll give some examples:

• Scissors are major causes of blindness among irresponsible children instead of "Scissors are a major cause of blindness among irresponsible children" (hopefully false)
• Cashews are excellent sources of magnesium instead of "cashews are an excellent source of magnesium"

The pattern seems to be cases when all things of a certain category are described as a certain way, where the things happen to be plural. As a native speaker, I know that treating the subject as singular sounds far more correct than treating the subject as plural. The sense is that we’re not describing individual items from a category, which would be plural, but rather the one category itself which is singular.

Is there a name for this type of grammatical construction and is there a general rule indicating in which situations it applies?