After living in Europe for six years now, my wife is still puzzled sometimes by the differences between Europe and North-America when it comes to the police: how they are experienced by the population and how they see and present themselves and which role they think they're playing in society. Big difference. I'm certainly over-generalizing but here, we see cops as approachable and helpful in general (with exceptions) while in North-America, at least my wife's impression is that of cops being mostly intimidating (again, with exceptions).
Of course, this is all complex and different social and societal aspects play a big role, such as e.g., the odds for a cop of running into an armed person. But when I read how the police handled the situation with the group of black trick-or-treaters, it seems so foreign to me now from a more European perspective.
I suppose accountability is always going to be an issue - who watches the watchmen? But it should not be - in a democracy especially, there should be functioning mechanism to prevent abuse of power, and that of course applies to police actions, too.
Continuing the theme, many police actually take indirect bribes. Look up Patrolmen's Benevolent Association cards and the like. There are different levels of tokens depending on connectedness and donation level, and you can find them openly discussed on police forums. I would guess the only reason we haven't developed a culture of on the spot cash bribes is that police would take the money and write you a ticket anyway for having insulted them. They have a psychological need to pretend they are on the right side of the law.