As the author says, most incidents of this kind, in most of the world, are protesters vs. police, and the police have .. a substantial amount of control over whether the situation escalates or not. Including just opening up with tear gas.

Conflicting football ultras is basically the only case where this doesn't happen.

(I've never been near a tear gas kind of event, but I did witness the Met Police deploy "kettling" for the first time in May 2001, close enough that if I'd not paid attention to the police lines forming up I would have been imprisoned uncomfortably for eight hours.)