the law seems to be "whatever the fuck the police say it is until a court tells them otherwise".
Yes, it has always worked that way, an arrest is not a conviction, it's an unproven claim. Body/car cams on cops may weed out some of the bad apples but here in Oz the cops are on the whole are decent people doing a dirty job and it should be noted that the vast majority of the body cam videos show citizens behaving badly and cops behaving with self-restraint and caution.
Having said that, when the cop's political masters start outfitting police stations like they do a military base and promote the regular use of guerilla tactics such as "no knocks" and swat teams in a residential setting, you are a fair way down the road to a police state, which is an entirely different thing to a police force. As the Stanford prison experiments so vividly demonstrated humans very rapidly descend into a violent master/slave relationship if the environment they find themselves in meets certain criteria (eg: Abu Ghraib, Nazi Germany).
It's a very deep seated behaviour in humans, we all have a ruthless dictator and a cowering slave with us just waiting for the right environmental triggers to emerge. Religious people have called it "good" and "evil" for millennia but incorrectly blamed it on angels and demons (as opposed to the naturally evolved behaviour of our species). Other than being aware we are all susceptible we can't do much to avoid such behaviour in ourselves, but we can set up political and social systems that discourage such environments from forming in the first place. The fact the US still embraces the death penalty and has such a high number of prisoners compared to the rest of the planet, is IMO a 'canary in the coal mine' for the emergence of a police state, statistically speaking the canary is dangling from its perch by one leg.