The difference between 911 and Japan's situation is that 911 happened in a more centralized area of a city. Unaffected greedy thieves who may live outside that area with low values could decide to go loot. In Japan, you would have to walk miles and miles to get to the flooded area you wish to loot (no trains / no roads)and chances are even those would be looters are still picking up their own mess in their own home from the earthquake alone or looking for missing family.
Honor also plays a big part.
I thought a good portion of New Orleans was underwater as well, due to the levee breaks, and the city basically being a bowl filled with water afterwards.
I'm sorry but that's funny right there.