As someone mentioned above, some people are protesting peacefully while others are committing crimes and rioting. The concern is that police, and now federal agents, aren't acting in good faith and aren't interested in distinguishing between the two. It's possible to be concerned about abuse of power by law enforcement - which has been rampantly on display these past few weeks - without supporting anarchist attempting to set fire to buildings.

If we could know for sure the police weren't going to abuse their authority, there would have been neither protests nor riots.