That's the problem though. If the Stanford Prison Experiment or that one about psychiatrists (Rosenhan) proved anything, those who have power are more and more tempted to use it the wrong way when things get more and more out of control (SPE), or are under the impression that they are infallible because of their position in power (Rosenhan, in terms of labeling people scum or whatever). There are plenty of great cops who don't succumb to the allure of that power, but there are plenty who do.
That manner of check needs to be in place.