The guarantee of free speech isn't a guarantee of free transmission. Trump can say anything he wants to anybody he wants (with certain minor restrictions, e.g. not crying 'fire' in a crowded theater.) The guarantee of free speech bans the government from interfering with speech, and the government hasn't tried to restrict Trump's speech. A private company may have its own reasons for blocking a person's use of their private means of transmission -- liability, among other things. Nobody knows what will come out of the court system, so if Trump issues a particular inflammatory statement, and that causes somebody to get killed, is Twitter then partially liable for damages? Well, who really knows how that would work through the courts? And even if Twitter prevailed in a lawsuit that went to the Supreme Court, how much would that cost the company? Do they have the right to protect themselves from such problems? I think they do.