In my algorithms class (and some others), our professor openly approved of collaboration on problem sets. He knew that students were going to collaborate anyway, so it may as well be encouraged and used as a pedagogical tool. The problem sets were more difficult because of this, but nobody was afraid to talk about them and help each other work through the proofs.

The midterm and final exam were in-person in bluebooks, and they were 60% of your grade. If you were just copying the problem sets, you would fail the exams and likely the class.