There exists an asymmetry in the dynamics of protesters vs police during a protest. Very simple:
Anyone can be 'planted' in a group of protesters to start stirring up trouble (agent-provocateur). Then the police have 'justification' to use whatever amount of force they think is required.
On the other hand, it's practically impossible for a regular citizen to be embedded into a riot police response unit.
Add to that the police have practically no real oversight and investigate themselves (assuming internal affairs counts as police).
One side can't fail (except morally), while the other side always will end up with the shorter end of the stick.