The problem is that cops (and prosecutors and judges) protect each other from accountability. If you saw your co-worker hit a pedestrian because they made a mistake driving would you lie for your co-worker and say that the pedestrian jumped out in front of the car? I hope not. Likewise, if you saw your co-worker intentionally hit someone with their car would you lie to protect them? If you did, that would make both of you bad, even though you never hit anyone with your car. That's what people are protesting: the fact that police can brutalize people without consequence.