Given how little their target market cares about being a "real" unix, a small amount is probably more than the benefit it brings in.
We are talking about it.
Talk is cheap.
In marketing, getting others to talk about you is not cheap.
I don’t know what to tell you. Pick your theory:
A) apple does it for no reason and it is a waste of money B) they do it because they are aware or benefits that outweighs the small certification cost