police have no legal requirements to know the law or what doesn’t violate it

Broadly speaking, that's true. Narrowly speaking, their job isn't to adjudicate your behavior, just to identify and report certain behaviors that might be criminal. Prosecutors are supposed to know the law and determine whether an actual violation has taken place, then press charges or drop them, accordingly.

But because going through the legal process is expensive - both in terms of time and monetary cost - any interaction with the police that results in an alleged offense sets an enormous burden on the accused. And because our legal system is heavily privatized, police bias can result in huge economic tolls put on people who "look suspicious".

This is why its so hard to get good fresh peanutbutter in America, today.