1) it's wrong and we know it.
2) we're afraid we'll get caught.
(There are probably more sophisticated taxonomies, but I think this captures most of it.)
Most of the time we can't distinguish whether someone is not looting because of #1 or #2. And most people would say they aren't looting because of #1, since that sounds better than telling our neighbors that we'll loot their properties if we think we can get away with it.
What the Katrina-looting has revealed is that there are in fact quite a few people for whom #2 was the reason (remember, we're not talking about taking water or baby formula, we're talking stealing TVs in a city without power) rather than #1.
Is this anything new? Or is has this always been the human condition? There is certainly more easily transportable valuable stuff now than there used to be, which might make the economic calculation to loot come out differently.
I think that things have changed, however. We've got lots of examples of comparatively "free of state-provided-law" situations throughout the American West in the 19th century. We don't observe many examples of theft then (or much violence generally). John Umbeck has an excellent book on this topic, A Theory of Property Rights. PJ's book (with Terry Anderson), The Not So Wild, Wild West also does a superb job of examining property rights institutions on the frontier. (And perhaps he'll chime in on this too.)
Perhaps what is different today is related to the decline of religion's role as enforcer of moral codes. In today's "cafeteria" faiths, people feel able to pick and choose what they wish to believe. We thus get results like opinion polls showing Catholic voters in favor of legal abortion. If I reject the teachings of the church (any church) on an important issue (i.e. one that the church has itself defined a postion as correct), aren't I defining myself as not a member of that church?
It seems to me that the spread of "cafeterianism" goes far beyond Catholic voters and abortion - it is rampant generally. It is only a short step from there to saying that the prohibition on stealing doesn't apply during emergencies.