Reason has an article on “Why Firing a Bad Cop is Damn Near Impossible.” In it, Mike Riggs explains about how difficult it is to remove a police officer, even when they commit crimes in uniform.
When police officers act like they are above the law, and when the law looks the other way, society loses its respect for law enforcement and—more importantly—the law itself. When the scales of justice are weighed unequally, then there really is no true justice.
For police officers, there is no incentive to be a good cop when you are surrounded by bad cops and the bad cops are never caught or punished. In these situations a bad apple really does ruin the bunch.