They aren't banning behavior, they're banning the expression of certain ideas. It would be one thing if the group was about beating up fat people, but posting

They aren't banning behavior, they're banning the expression of certain ideas. It would be one thing if the group was about beating up fat people, but posting comments online about them is clearly under the ideals of expression. If they want to ensure it's behavior and not ideas they should get a judge to rule on whether the expression would be constitutionally protected. And yes, as a private entity it is their prerogative to ban certain ideas on their site, however, they should just be honest about it, they are limiting speech in a manner that would likely be unconstitutional if they were a government entity, aka. they are abridging freedom of expression.