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.
All of those situations still have the problems associated with deploying the military against civilians: The military isn't trained to carry out appropriate law enforcement. It compromises trust in and support for the military. It tends to be used disproportionately against the left rather than the right. It is easily abused to suppress dissent. Etc.