Honestly, I was more thinking about the layer of abstraction above individual communities like OrganicMothers.com and AnythingGoesMothering.com - the one made up of shared services like Reddit and Facebook that are used to establish communities, since they're harder to avoid. But even at that level, speech-restrictive websites can certainly be worrying. For example, anti-vaccination ideas are likely to be popular with your hypothetical organic mothering community, and obviously they are also likely to attract a lot of anger. So the community would certainly be a lot friendlier if they banned criticism of mothers' choices not to vaccinate their kids. Can you perhaps see how this might not be socially desirable?