I think the principle is pretty broadly recognized. As I understand it, international law would treat them as unlawful combatants. Use of irregular troops against a civilian population falls pretty much smack dab into the middle of what most people recognize as terrorism.

The only problem is that international law doesn't govern how a country conducts activities on their own people. But it reflects a widespread consensus nonetheless.

Note: I'm not accusing the police of terrorism -- that would be pretty massively inflammatory and not my intent -- but simply giving examples to support a widespread discomfort with using non-uniformed forces.