So maybe when it's one of their own they can work the case, but when one of their own has a relationship with or knowledge of the victim or suspect they get removed from the case? Sheldon knew the girl that got killed on the subway party and got removed...Danny had a relationship with the Tanglewood Boys and got removed. If I remember, Mac did the 333 case sort of off the books because Sinclair was asking about his whereabouts and everyone covered as best as they could. Aiden's death did seem to be an anomoly because they all knew her. I guess the fact that she was one of theirs made it okay. I just don't know. The reason they all got pulled off the case when Hawkes was arrested for robbery and murder was because Hawkes was the suspect. Makes sense to me.