The cops are watched a lot more closely than everyone else, and you have to apply and pass some evaluations to get in, so not just any idiot can be a cop.
That really sums up why people can be idiots on the server though. There's no way to hard code the game to keep people from just running around and killing people, so when some jerkoff goes apeshit like that, all you can do is /report it to the admins, and/or call the cops, and hope they're around to deal with them. There's usually some cops/FBI on, and an admin or two at least 12 hours a day, so it's not usually too bad.