Posse Comitatus gets winked at in cases of urgency. I was an Air Force Security policeman at NORAD some 20 years ago. We received calls of gunshots on the Cheyenne Mountain access road. We went to check it out and figured there were some poachers hunting on the private property that lined the road. We called up El Paso County Sherriff and a deputy came up to check it out. While we were standing there a couple of shots were fired and the bullets whizzed by us. We hit the dirt and the deputy took us with him to go flush out the poachers. It was the only time I ever got shot at in the military! When the deputy told us to come with him, we said we couldn't because of Posse Comitatus. He let us know in no uncertain terms that we were to come with him. When the bullets are flying, you worry about the rules after you deal with the threat.
That's a good story--when the bullets are flying people start bending the rules. I'll tell you one thing...the right thing to do here, both morally and politically, is to bend the rules and use the military for rescue operations if necessary and to scare the hell out of rioters with the mere appearance of heavy machine guns.