Of course the constitution does not protect violent speech, but that’s not the point. By designating antifa as a terrorist group, Trump can prosecute someone who only joined the group, but didn’t commit any violent crimes. Hate speech is explicitly condoned in the constitution and for good reason and designating antifa as a terrorist group would undermine free speech. By your logic, white nationalists who are many times more dangerous than antifa would be a terrorist group, but they aren’t. That’s because simply joining a hateful group doesn’t make you a terrorist. This also indicates that trump’s move is not a principled stance against terrorism, but a political one against specifically left wing hate groups while brushing aside right wing ones.