The people they are policing aren't enemy combatants, but their fellow citizens -- and, even more significantly, their employers. A combat-like mindset on the part of police turns fellow-citizens into enemies, with predictable results.
I sometimes think the turning point was marked by the old cop show Hill Street Blues. Each episode opened with a daily briefing before the officers went out on patrol. In the early seasons, Sergeant Phil Esterhaus concluded every briefing with "Let's be careful out there." In the later episodes, his replacement, Sergeant Stan Jablonski, replaced that with "Let's do it to them before they do it to us." The latter attitude is appropriate for a war zone, but not for a civilized society.