- First, public prosecutors, district attorneys, and such are reluctant to bring the most serious charges that they could against a police officer. The reason for this is that prosecutors need the good will of the police collectively because they know that they are going to need the police to testify on their side, the prosecution, in future criminal trials.
- Second, judges and juries are predisposed to be sympathetic to the police, to not only give them a lot of deference and sympathy but to believe their testimony. Again, in the typical or common sort of criminal trial, it's a case of the "good guys"--the police, representing law and order--testifying against defendants who, despite the supposed presumption of innocence, are likely to be seen as the "bad guys."
- Third, police are extremely unlikely to testify against other police. They have a solidarity with one another such that they will lie and conceal or distort evidence in order to help to acquit another police officer. And police hierarchy, police unions, and "police spokesmen" (really no more than public-relations professionals) universally defend the police and are totally unwilling to ever admit that a police officer could be at fault in any particular case.
An interesting statistic: for the latest year for which statistics are available, police in