If the server is hosted in the United States of America, then they're protected by the First Amendment. However, there is no US (hopefully no state-level) law preventing people on the internet from criticizing the site.
Most laws and amendments only apply to public. Saying racial slurs in public can get you in trouble, in your own home won't. Because this is a private forum baddy is the judge, jury, prosecution, land lord, god, law maker, president, king, ect and may enforce his own rules as he sees fit.
Also those like Piers Morgan that use the first amend to censor criticism are spineless and selfish men. The First amendment can both ways, not one.