If they want to be totally hands off, they can do that. But if they want to set a tone for how people use their service, they can do that too.

Reddit is a platform for discourse too, and they've decided to evict certain communities from their platform because they don't match Reddit's goals and values. HN is a platform for discourse and there is a reputation system that controls how people interact with it. Facebook is another platform where users can decide who they interact with, and how.