Wasn’t the military hanging around ground zero after 9/11? Could explain why there was no looting.
Not to mention, as horrific as 911 was, it was in a relatively small and contained area with functioning services all around it.
911 is just not a good analogy. A better analogy for NY would be the 1977 blackout or the 64 riots.
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.