I wonder if some students have a problem with study groups because they can't find one to join. So they accuse the group of having an unfair advantage ("cheating"). As long as HW is not weighted as much as the tests (like 10%-90%, 5%-95%) I don't see a problem with groups. You can't make things "completely fair" anyways. Also, in the real world, people collaborate on problems all the time so it's good practice to get students working together. The point of a course (you'd think) is to teach the material to the students and if study groups contribute to this then all the more power to them.