You're making a much too extreme dichotomy.

The situation is that American police are generally there to enforce the law, generally follow policy but, depending on area, are infested with a minority of types who use or threaten violence whenever it makes their job easier or simply serves their ego. And these forces also tend to have a much larger group who won't say anything about the overt criminals - out of conformity, alienation from civilians or because the intimidation factor is convenient to them. This situation, that is effectively well publicized at this point, fundamentally changes the way police and civilians relation because it goes a lot of police actions a threatening quality and so, as I said earlier, it's hard to have question appear "optional" to the average person stopped by the police.