People love to blame and demean others when they feel they have been wronged. Doctors are accused of being in it only for the money, lawyers are all bottom-feeding opportunists, teachers teach because they can't function in the "real" world, etc. They do these things because it's easier than thinking logically about either the wrong they did or the faulty system that the individual officers, doctors, lawyers, teachers, etc. have no say in.
McCoy says these things about police officers because they are the easiest of targets. There are enough bad ones to make for decent headlines, and those who do good go largely unnoticed because that's just "what cops are supposed to do". Almost anything they do can potentially be turned against them and the reactionary public allows police officers to be thrown under the bus moreso than any other profession. The stress of the profession is ignored and people scream in anger when a cop harms someone who was trying to harm them.
Police enforce the law, some better than others. The problem is the law can fail, and when that happens even good cops get the blame. Other times everything is done by the book...and the cops still get blamed "because dey was jes' keepin' us down, man".
