There are plenty of cases involving officers reporting the misconduct of other officers. In any case - unless you're arguing that the police actively recruit sociopaths, you're simply bumping up against the less endearing aspects of group psychology - people within a group tend to identify with the group, and actually perceive its members' actions in a more positive light than those from without.
( Happy Phantom has been to Hastings, Brighton, and Eastbourne too, Fri 6 May 2011, 17:26, 1 reply)

Yes, and the police DO get held
to a higher standard these days e.g. the scum who was coercing female suspects (many addicts/prostitutes) for sex, often violently. CCTV from cells contributed to his conviction, demonstrating that his fellow officers acted against him. He got life, which I think is rare for rape.
A dishonest officer i.e. taking bungs from criminals is likely to serve time A civilian taking bribes from clients would be sacked.
Obviously all the cases that get covered up won't be known about by definition, but plenty of employers would equally seek to avoid negative publicity,
(sarcastic fringehead feet of clay, buns of steel, head of lettuce, Fri 6 May 2011, 19:28, closed)