[potato3732842]

Every institutions #1 priority is to amass power and perpetuate itself. Of course the violence arm of the state engages in extralegal surveillance and extralegal intelligence gathering and extralegal violence to these ends of course it curries favor with the arms of government it seeks to ally itself with by offering to do so on their behalf on a limited basis.

Speaking about the police specifically, or perhaps more generally the executive branch of the state, the only thing that checks this power is the risk of them doing something that so threatens the populace or other institutions that they ally against them. The point at which this check starts becoming a realistic possibility explains the variance in volume and forms this bad behavior takes from country to country, culture to culture, etc.