There are two reasons for this, both having to do with the college's educational mission.
First, colleges want quality discussion posts because discussion is part of learning. Students are expected to truly think about the material that they are learning. They are expected to show evidence of having done so by posting high quality posts as opposed to perfunctory posts that show no signs of thought.
Second, colleges need to ensure that all members of the class will feel safe participating in class. They need to ensure a positive and supportive learning environment. If people post inappropriate things in their discussions, it can make it feel less safe for other students. They might feel that they will be insulted or criticized simply for trying to get their opinions across.