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.
The police and prosecutors need to work closely together to function. It's unreasonable to ask the prosecutors to then also prosecute police powers. It's biased and unjust—even when the actors are all doing their best to act in good faith.
We currently allow PDs to investigate themselves, which is a mockery of justice.
It ends at subpoena and arresting power. The courts would mediate disputes as they already do.