Great question! The answer that I always come up with is that they may think they're above the law and don't have to follow the same rules as we do. I work right by the police station of my city and I see it all of the time. So, I know they're not hurrying to a cal they just received. They are just trying to hurry back to work so they can go home.
Let's not make generalized statements. There are police officers that do violate the law because of their position. But not all, and the few that do stick out and give the ones setting a good example by driving within the limitations a bad representation.