You underestimate the power of racism, homophobia and other things that drive some people, plus the feeling of having power over other people (and also, if a crime has been committed, the drive to make culprits out of innocents if police cannot find the real ones).
Sure, the US has some amazingly broken systems that seem to foster endemic corruption, but even without them it’s not enough to guarantee that a police force behaves appropriately (we just had a case in france where at least two people have done 1.5 years and 4 years in detention based on a made-up testimony forced by police officers caught on video, and the police unions and the far-right are still screaming that justice is too lenient and the liberation is a scandal).