Of course, those that break the law will always attempt to conceal that fact, out of the interest of self-preservation... criminals hiding behind a badge are no different. How do I explain Chicago, or Atlanta? I don't. They had to have someone on the inside protecting their interests. That, or their IA departments are horribly and almost criminally negligent in their duties.
And what does that mean? It means that a disturbing number of large cities' police departments are incapable of policing themselves in any effective way.
I'm not talking about ONE cop doing something wrong. It's literally IMPOSSIBLE to stop that. As hard as I try, I can't really think of a way to blame the North Canton, Ohio PD for Officer Bobby Cutts, Jr. murdering his pregnant girlfriend and enlisting a friend and a relative to assist him in covering up the crime.
No, what we're talking about is large GROUPS of police in certain departments (that we know of) engaged in well organized, pervasive and ongoing criminal conspiracies, COMPLETELY unhindered by their superiors, who may or may not be personally involved.
These CONSPIRACIES have called the SYSTEM into question and its commitment and ability to protect the rights and lives of the citizenry.
That is why we have those "outside" agencies... To help ensure that those entrusted with upholding the law aren't breaking it themselves. And, by the very fact that we know about the cases you mention... They appear to be doing their jobs. How many do we not know about? No one can say with any certainty.
That's a tacit admission that we CAN'T trust police departments to police their own. The Johnston murder was solved PURELY by CHANCE. I don't know whether the breaking of the Danziger bridge case was as wildly capricious as that of the Johnston case.
The truth is that many of us seem to be trusting to luck in regard to supervision of local police. Kathryn Johnston and the Danziger bridge victims are proof that that isn't good enough. I don't know what SYSTEMATIZED oversight the FBI exercises over rights violations by local police, but clearly it sometimes is lacking. The Chicago PD AND the FBI somehow managed to not notice systematic torture (REAL torture with beatings, electricutions, burnings and suffocations, not harsh questioning) of suspects for more than a decade... until the statute of limitations ran on all of the perpetrators.
Probably what we need is a PURPOSELY ORGANIZED element within the FBI to handle these issues. I don't know if such exists, but if it is, it sure doesn't seem well supported.
I was a LOT more concerned that I would have a negative encounter with the Chicago PD when I was there last week than I EVER was of being attacked by some alleged "Christian militia".