My suspicion is that after this cop was shot this morning the shoot to kill order was put into effect for both the cops and the National Guard.
Having been in the National Guard and being close to a number of police officers (including my brother) I can guess what orders were (or soon will be) issued - Use the minimum force necessary to protect yourselves and innocent citizens. Of course, everyone understands that means the same as "shoot looters on sight" but we are not allowed to actually SAY that.
Personally, I am a traditionalist. Tall tree, short rope, problem solved...