Posts are "public" (i.e., viewable by everyone in class) and are typically accompanied by the poster’s real name. And learners know that posts are viewable by the Community Owner even when they post anonymously. It would be fairly unusual for a learner to stand up in a classroom and start yelling at another learner in front of the instructor and the rest of the class. Similar social pressures dictate acceptable behavior within a Yellowdig Community, and instructors can easily set the expectations of the community and re-iterate them as necessary on the rare occasions where a problem would arise.
Some instructors also voice concerns about post quality or learners getting points for leaving short, thoughtless commentary. Their concerns are understandable, but social points—points for receiving comments, reactions, and accolades—reduce students' incentive to post "junk content", since students who post junk ultimately have to work harder than students who do good work and consequently receive many social points. These social points simultaneously dissuade anti-social behavior and "trolling".
Why? You shouldn’t expect disruptions; Yellowdig is remarkably non-toxic. To put it bluntly, there are real social and academic consequences for acting like a jerk within a learning community when grading is enabled and points are on the line. Yellowdig Communities rarely require policing, and in the event that correction is necessary, there are ample mechanisms in the forms of post flagging and word blocking.