Cops are not much different than soldiers. Some join out of a moral conviction, to serve their country or promote justice or whatever. But of those, few see it as a principe which must be upheld at any cost, so they will let the dictates of the job slowly whittle away at their principles until there's nothing left of them. For soldiers, it's probably around the 3rd deployment straight of guarding poppy fields. For the cops, it's probably seeing that violent felon they wrestled to the ground released 10 years early. But either way, most of the idealists wind up joining the ranks of those that never cared at all, which is making up more and more of the new recruits.