Not that I want to sound like a voice of reason, but in U.S. jurisprudence the most egregious acts have historically been forced to be tolerated, because at the time they occurred there was no law against them.
44 AMP makes a good point, that after hurricanes came through there were some who used chainsaws to try to perform contructive acts, while others went on looting sprees because everyone else had fled their homes, and they were easy picking. No one was even around to witness the acts.
Its important to remember that law enforcement has a significant amount of discretion regarding whom they choose to apply laws to.
Occasionally a law may be needed so that it may be applied against genuine criminals, when there is no intention of stopping every car going to the 7-11 for milk to see whether there is a handgun in the car or not.
If there is no law against a behavior that a criminal might exhibit during an emergency situation - which on it's face might not appear criminal from the point of view of the law-abiding - (such as walking down a street carrying about 10 long guns recently liberated from some empty house), there may be no particular law that the police might use to charge such a person with breaking.
I agree completely with Pax that laws must be carefully written to protect citizens' rights. But its important to realize that laws apply both to those who obey them as well as those who are disinclined to do so. If you want to charge some lowbrow azzhat who is taking desperate advantage of an emergency situation, there must be some law in existence that allows this disenfranchised citizen to be stopped and held.
Laws work both ways, which is why they must be carefully crafted.
That said, I can't figure out the bit about ammunition either.
(OTOH, most of us are going to have a fairly substantial stock of ammunition available anyway. Could it possibly be that the ammunition sales ban is also aimed at those who may have suddenly acquired a firearm by dubious means?)