It's not really about his own culpability - Jim is saying that it sets up an easy defense for minorities charged with crimes. There's caselaw that says that it is relevant (a history by an officer or police leadership) and can be used by the defense to argue bias against the suspect.
So most well-run prosecution organizations (DA+police) are keen to avoid hiring law enforcement with that kind of history because it just puts a drag on your ability to get convictions.