That there are some bad cops out there, there is no question. They should be discovered, rooted out, and prosecuted. However, I can with complete confidence state that the vast majority are caring, concerned professionals who serve with pride and a desire to make their society better than they found it. You, apparently, cannot say the same. To make a general statement that police officers lie on the stand every day, is to make a sweeping generalization.
Police officers are just people, no worse and no better than others.
They have, however, dedicated their lives to the pursuit of justice. And while a few may lie on the stand, most do not. I will not try to tell you of their honesty because it is readily apparent that you will not listen. You are incorrect that there are no laws to make sure that police uphold a person's rights. There are several federal statutes that govern the conduct of police officers, as well as continued judicial review by the Supremem Court.