No one has ever been killed by repeated poking either, but go to your local biker bar and start poking one of them repeatedly and see what the cops say when they're wheeling you into the ambulance; "well, you probably should have stopped poking him." The courts have repeatedly ruled that speech that is intended to provoke immediate, violent action is NOT protected by the first amendment. Whether that's the case in this specific incident is up to a judge to decide, but based on the evidence I've seen, it seems the protestors were actively trying to start a riot, in which case, their speech is not protected.