If someone put legible graffiti on your house, and you weren't allowed to remove it, and it thus appeared that you endorse whatever the graffiti is saying (not

If someone put legible graffiti on your house, and you weren't allowed to remove it, and it thus appeared that you endorse whatever the graffiti is saying (not to mention having to encounter it yourself all day), then that would indeed be compelled speech. But we're not really talking about that. A site not removing a post is not an endorsement of the post, especially if they are legally required to not remove the post. I'm not saying that prohibiting removal of posts is necessarily a good idea. It can certainly be argued that the company providing the forum ought to be allowed to do whatever they want. But, for better or worse, "people/companies should be able to do whatever they want" is not a principle of US law. As far as I know, US states can make it illegal for anyone to cook hot dogs after 3pm on Wednesdays. If challenged in court, I think they don't even have to provide any rationale for this - it's enough that it is a law properly passed by the legislature. And of course US states have numerous less nonsensical laws regulating businesses, including things like prohibiting businesses from engaging in racial discrimination, or listening in on telephone conversations.