David A. Harris in his book Good Cops: The Case for Preventive Policing explains the underyling reason for the “us versus them” mentality that is so prevalent in the world of law enforcement.
No phrase characterizes the thinking of police officers more completely than the cliche “us versus them.” Policing is indeed a world apart. Police officers deal with the worst, most troubled, and least caring elements of society. They see humanity at its lowest. Much more than the rest of us, they face the likelihood of meeting or using violence every time they go to work. Most shifts may be uneventful, with nothing more than traffic tickets to write, reports to take, or disgruntled neighbors to calm, but officers are also cursed at, spat upon, yelled at, and accused of things that most of us can only imagine. It is not
surprising to find that police culture is insular and suspicious of outsiders. Only a brother or sister member of the force can really “get” what a police officer has to go through; when a police officer is in trouble, it will inevitably be a fellow cop who backs him up, not a civilian. (Page 133)