"Regular military is forbidden to do law enforcement, even under martial law. The Guard does it. Special orders have to be given by the president, and only under very specific conditions, which have not been met."
Last night the National Guard was begging for law enforcement assistance at the Superdome because of the 'volatile' situation there. The NG stated they needed protection.
So, in reality the NG is not performing as a police unit in this situation yet or is uncomfortable with that role.
On a side note, a news graphic this AM stated that 8 looters had already been shot.