While it's true that not every cop is actively corrupt, or violent, they have this whole "Blue Code" thing that means that if you ever report a coworker for being corrupt or violent, you're out. The cops who enable bad behavior, and perpetuate the bad system, are just as bad as any other corrupt or violent cop. So yeah, they certainly can all be bad.
The idea behind the phrase is that cops are either bastards or they sit by and let other cops be bastards and are themselves bastards. Any cop who doesn't sit by and let other cops abuse citizens isn't a cop for very long.
