Why do cops feel like they are the only public servants that should not be questioned?

Or be called out for illegal activities that they perpetrate against their employers, The American people? For instance, some good citizen sees a cop abusing his power, and being that this taxpayer pays his wages, films the event so as to have proof so this cop can be disciplined or lose hos job. The cop sees this man filming his ill concieved activities and physically assaults the person. Why is this?