This makes sense, since to the best of my knowledge the average guardsman doesn't have the authority to arrest anyone.

Probably true. They arent there to arrest. They arent there to kneel on anyone. Odds are they dont have a loaded magazine in those weapons. There are there to show them "enough, we will draw the line right here. Then we will move this line. Then we will institute a curfew. You will have us stand on your street corners. We will leave when you can behave."