"Bears are drinking milk" is an odd sentence in English because of how we use articles. The implication is that bears in general consume milk although they previously did not.
"The bears are drinking milk" would be a better translation of how this would be used, though I know Duolingo likes to use ето/ета/етот/ети to specify definite articles when translating from Russian to English and vice versa. Therefore I think the best translation would be "bears drink milk" as this makes the most contextual sense in both languages.
Yes, usually duolingo will accept both. However, sometimes it may decline one when one or the other would make little sense - for example лошади пьют would almost always translate into "the horses are drinking". The reason being, "horses drink" isn't something you'd ever say in basically any situation, so although it is correct, it's probably better to mark it as incorrect in a learning environment.