The people who did the work obviously think it's fine, otherwise they wouldn't have licensed it under such a permissive licence.

Certainly. MIT/BSD more or less says: steal away (but retain this license notice). In some cases that's desirable.

Is it stealing if somebody gives you a present and you accept it?

I am not going to debate whether it's "fine", but it's certainly not stealing. If I make someone a beautiful necklace and they sell it in an auction, I might wish they hadn't done that with my gift - but they haven't stolen anything either.