Any private non-government entity can censor whatever they want to. The US laws only forbid government censorship and even that has a few limitations. You can't threaten the president, yell fire in a movie theater, lie to the cops or make phony 911 calls.
But we also get so uptight about our rights that most of us forget our obligations to our fellow humans. Words can cause problems and incite hatred and fear with people. You can say what you want but most people are not very good at taking the heat for doing so.