I could see where you would come from with your protest idea.. and it is true, it could happen.

Though primarily police forces are brought in to "keep the peace" so to speak, I've seen several protests take place and the police involvement is to ensure safety among the protesters and the other civilians in the area. In Toronto, there were several protests in which police blocked off roadways to sort of rout the protesters, but also to keep them safe from traffic and any conflict with people against the protest. The main goal for the police force is to protect and serve, offering protection for the public and serving the public, which is what their involvement in protests would be.

If the protest turns violent which it is possible it could, a stronger force would be utilized to help protect others and contain the conflict to a certain area. Realistically if violence occurred, as a citizen not participating in the protests, I would want the police forces to use some form of force or tactic to contain and deter the violence.

If the deterrent is not effective, the common riot police would be utilized with tear gas and riot gear to help contain and stop the problem from continuing.

I see where you are coming from with the snowball effect, but there are various systems and processes put into place for these exact situations in which a quiet protest turns to violence.

The police are there to protect and serve the public and they will do so