Who decides if it is a peaceful protest?

Ultimately the platform, and yes there are huge grey areas. I don’t think this instance lands in one of them and that’s why Facebook isn’t even claiming it does.

It seems like you’re trying to avoid dealing with the substance of the issue for some reason.

Usually local police if they are on the scene. If they start seeing violence, they start telling people to disperse, followed by tear gas and then rubber bullets.