I think you might find that it's not all that difficult to find LE officers that will testify against another officer that has acted badly, although this can be a regional issue. Cop culture varies quite a bit from one part of the country to another. In the Northeast, cops tend to be very protective of one another and an officer that comes forward to report misconduct or grave incompetence is likely to be ostracized. However, in the Northwest, the standard leans more towards self-policing (no pun intended). Take a look at the ethics bulletins published monthly by the Oregon Department of Public Safety Standards and Training. DPSST de-certifies a number of officers nearly every month, and then publishes their reasons for doing so. This sends a clear message that misconduct will not be tolerated. If an officer is de-certified, he can never again work as a law enforcement or corrections officer in that state, and it is nearly impossible for him to obtain similar work elsewhere.

It's difficult to change the nature of a subculture such as that of police, but I think it is happening slowly. Even now, I don't believe that the police culture is as rigidly self-protective as you portray it to be.