This doesn't seem to hold well in practice.
> The moment protestors cross over to vandalism and violence, they're rioters and not mere protesters.
And this seem to be usually applied collectively (in obvious opposition to basic human rights) in practice. Often in cases where the violence wasn't done by the protesters and even in cases where the violence was against the protesters.
Also, police has a really shitty history of capturing and holding into account the actual violent people. They usually go free for turning the next protest violent too.
And both happen again and again, all over the world.
Also very possible that the state actors are the ones beginning the violence which is well documented to have happened during the Troubles in Northern Ireland.