The real problem is that your average street cop deals everyday with the scum of society. They seldom come into contact with the vast majority of citizens that obey the law and just want to go about their business.
Because cops are surrounded by scum all day, every day they end up believing that that's all there is and when a decent citizen runs into a situation where he needs a cop, the cop treats him like he treats the people that he is used to dealing with.
When cops walked a beat and knew everyone on that beat, these kind of problems were much less prevelant and cops tended to be more even-keeled.