Actually it’s damn near impossible to subpoena rogue cops, due to union laws that protect them from consequences for wrongdoings. That is a demonstrable fact. This is often aided, as mentioned, by internal social pressures within police departments – and there is plenty of legal precedent and evidence to show that this is far more common than most people realize or believe. (The “few bad apples” adage is often brought up by every side of these debates, while ignoring that “a few bad apples spoil the bunch.” Even if these problems are limited to a few bad apples, the fact that those apples cannot be removed except in the most extreme of circumstances means that the rot will continue to grow.)
I have personally witnessed—in public, and in training scenarios – far too many instances where police approached with outright hostility, and the threat of coercive violence, even when the situation does not call for it. This triggers human survival instincts, which can manifest as resistance – which the officers can then use to justify actual violence. I have seen this explained quite explicitly to recruits, giving them permission and encouragement to scream at and threaten anyone they choose to at any time – a behavior that’s protected by the aforementioned laws and culture.
So while I understand that you, personally, might place your faith in a policing system that you believe to be just and fair, I hope you will recognize that there are many people who do not share that faith, for very valid reasons. And that this creates a fundamental flaw in relying on that system as an arbiter of justice. Remember: police departments were not invented to serve and protect “the people” at large. They were specifically created to keep down former slaves and factory workers at the behest of the wealthy. This is historical fact, and that power dynamic remains baked into the core.