> Where did this adversarial attitude to law enforcement come from?
They screw up very frequently. Sometimes maliciously, sometimes through incompetence, sometimes both. I can't convey the depth of this in a small comment box, but there's abundant evidence around on this topic if you care to look.
Overall, even when you're talking about legitimately designated authority given to a person ... it's VERRRY easy for a human being to screw up and get it wrong, and it has huge impact over the lives of their targets. Needs to be approached by the authorities with extreme caution. In practice, probably many of them aren't aware of the weight of their actions, or don't care.