Right, there is a monopoly on the perceived legitimacy of violence by police. As you mentioned, a government-sanctioned monopoly prevents any sort of feedback mechanism from being effective. Without competition and with no way of opting out, there is no need to be accountable to your 'customers'. If someone is a customer of your product whether they want to or not via coercion, there's no need to please any interests outside of the internal agenda.

There is also an argument no moral cops exist because a moral cop would whistle blow an injustice. Doing so would often lead to them quitting or being fired for violating "the blue shield". This would mean any cop is immoral whether in action or inaction. Under this thought, the only potentially moral cops are former ones having realized their wrongs.