A couple of reasons off the top of my head. They investigate their own crimes that they commit. The fines are paid by the public and not the officer themselves. They should be made to have insurance like Doctors. The Judicial system let's them off all the time with probation and shit when they do get charged. Last but the most important the Damn Police Unions have to much power and influence. Politicans on both sides of the aisle love Police unions, but shit on all the other unions.