You are saying "there exist police departments which are friendly with the protesters".
The thread you are replying to isn't talking about protests; it's discussing the absence of evidence where police officers actively intervened and stopped the excessive use of force by a fellow officer.
In the USA, a civilian doesn't have the right to defend themselves from an officer under any circumstances in most states. A very few states allow you to shoot to defend yourself (even from officers who don't present themselves as officers) under very narrow circumstances. Any way you cut it, police are given benefit of the doubt when interacting with suspects but the cheaper internet-connected cameras get, the more evidence that we should probably revisit that long-held doctrine.
On the whole, the cops have been insanely brutal.
I cannot believe how far people will go seeing what they want to see.