Per Supreme Court decisions over the years there are
limits to free speech. This guy is entitled to say what he wants about Mohammed and wear a costume of him but as it stands he has no protection under the law of the United States, in fact he should get in trouble for it. If this dummy were to get gunned down the assailant would get no more than a second degree murder charge.
Muslims take insults to Mohammed very seriously, enough to murk you outright. Unless you think you're good to someone dead, keep your thoughts on Mohammed behind closed doors. Ya'heard?