But since it was on Iraqi soil and against Iraqi's, you're looking at Iraqi law here. Meaning a Iraqi police officer would have to make the arrest, meaning that there is no way in hell these guys will get prosecuted, sadly enough.
And Rictor: Read the Geneva Convention, there are a few clauses calling anything properly uniformed and adhearing to the laws of war a military, but it's all rather shaky once you get to small groups of mercs.
And yes, I once did spend a night reading most of the Geneva convention. Twas during the invasion, actually, since I was checking about the video thingy,
As for arming Kurds and letting them police Iraq, not the best decision they could make. But right now, whoever you give a gun in Iraq, you know there will be some grudge left from quite a few years ago. You can't go right.