A lot of that comes from the nature of the police force in general: police come from two segments, those who want to make the world a better place through enforcement of society's laws, and those who wish to have power over others with their position.
The former group are already committing a fallacious appeal to law, and the latter an appeal to authority. Neither group has a good justification for enforcing the law as it stands.
Sure, you get the occasional social contract theorist, but they don't last long surrounded by either of the other fractions.
This you end up with the assholes, and those who protect them because they see the law as more important and assume the failure of officers in public light damages the rule of law.
What would actually fix the problem, requiring all officers to be philosophically literate moreso than legally inclined, would never fly, because then officers would do what is right for everyone rather than merely what is right for those in power (keeping the poors down).