Which, I think, is part of the problem. Prosecutors, like defense attorneys, are officers of the court; they're supposed to be part of the "court's team," with a mission of finding a just result, not a part of the "police team," with a mission of putting somebody in jail. More than a few fail to make that distinction, though, either by honest misunderstanding or by ambition at higher office.Not all, but a lot of prosecutors are reluctant to tackle the bad cop issue in criminal court, particularly when their fellow officers are part of the prosecutor's team for all the other cases in their jurisdiction.
That's definitely not a police problem, though.