So, Reese Witherspoon got locked up for disorderly conduct, i.e. mouthing off to a police officer. Of course, we all acknowledge that telling the po-po to eat shit is both totally legal AND a one-way ticket to jail. She kept getting out of the car to spout off, and there's always the "officer safety" catchall justification, but the basic standard is arresting someone for legal conduct that the officer doesn't like.
Remember the Henry Louis Gates debacle? I think it was less about race than the average officer's intention to use police powers against anyone they feel disrespected by. This is my pet peeve: that if you tell a cop to throw his weight around and dominate and threaten people who haven't committed any crime, then he'll know EXACTLY what to do. Tell him to go solve that rash of break-ins, and he's buffaloed.
To finish, I'd like to mention that I've known a few police officers personally, and they're uniformly horrible, horrible people. Sick bastards without any sense of morality. Within ten minutes of meeting him, one was going on about the multiple firing-worthy offenses he commits DAILY, and it's just like "Oh, wow, another bent cop."