Violence conducted via the military or police, according to regulation, is lawful.
But violence conducted by citizens, or by members of the government or military that is not according to law/regulation, is not lawful.
I'm not saying Twitter's drawing the line exactly right, but it's somewhere in the right vicinity.
I think Trump's saying that if things get out of control, law enforcement will start shooting. If I understand your post correctly, this would be lawful...