There is a LOT of social pressure to shield a bad cop, shunning, no one wanting to go on patrol with someone they "can't trust", colleagues leave the cop bar when you show up, etc. Maybe one in 10 of that 85-90% can stand up to the possibility of shunning. A cop who would encourage or even participate in shunning, or other acts of retribution against someone who exposes a bad cop, is himself a bad cop, even if he commits no other act of personal corruption.