Army retiree here. I suspect these are for getting troops around the city for logistical purposes ie. shift changes, food, airport runs if needed. You cant manage transportation for 25+ thousand troops spread across a city by giving them all rental cars and keep any sort of management in place. I highly doubt if there were going to be prison buses brought in that they would a) be brought in during broad daylight or b) be brought in before they were required. I'm not there so I don't know for sure, but that's what my experience tells me. Just my two pennies.