I’m not a lawyer, but in at least the US, that would make sites publishers. They’re not. They’re basically hosts, which, as I understand it, they would otherwise be legally responsible for the content that users post on their sites. No company wants that, but will at least try some form of rules and/or moderation so as to have a defense of good wiill, should they ever have to appear in court due to content users posted on their site.

Edit: clarification

How do you stop an elephant from charging?