Cashews are excellent sources of magnesium?

Question

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?

There is a fake rule that subjects and subject complements must match in number. See Subject/Complement Agreement..., especially the comment by @tchrist (for a concise overview): "The only agreement rule is that finite verbs must agree with their subject. Period. There is no such thing as “subject–complement” agreement in English, be it grammatical or notional. The problem is the horses is perfectly normal, standard, and correct in all possible ways..." — Tinfoil Hat, Jan 18 '21 at 22:02
Does this answer your question? Singular noun objects of plural subjects — niamulbengali, Jan 18 '21 at 22:03
The problem is that scissors do not form many causes, and cashews do not form many sources of magnesium. — Yosef Baskin, Jan 18 '21 at 23:26

Cashews are excellent sources of magnesium?

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