I believe it's the primary reason for the complete apathy law enforcement shows towards de-escalation and self-restraint in general.
Why bother behaving when the standard for prosecuting you is so high as to be laughable?
So a pretty reasonable question you'd ask in the rest of the world is: Why aren't these cops prosecuted for excessive violence? The use of civil rights lawsuits in the US was a workaround for an already broken legal system that doesn't punish wrong doing by certain people.
Until the concept of "Internal Affairs" dies and we get an actual independent investigative arm or some other strategy that will continue
The only time an outside investigator is called in, normally is because someone dies AND the public is upset about that death.