Conceptually, you are right that it’s a theft motivation for most, but it is without any damage to the owners (or even insurers) at this point. If those things aren’t taken now and used, they’ll be trash in a week or couple days when cleanup comes through. I doubt that is in the minds of the looters, but it was a complete inventory loss before they showed up.
We also don’t know if some of those people’s houses and supplies were wiped out. This storm was brutal and if that building took that damage, houses are likely worse. Point being, at least some of the looters might already be out of supplies.
But even if these looters have mapped out the inventory impact, insurance claims,and potential waste of otherwise useful goods, it's still theft. And as you said, I doubt the people running into a Dollar Store after a storm are bright enough to even think about that. So in essence, we arrive at the same conclusion.
To your second point, that's where I gave myself a little wiggle room on my opinion. I could see if we were weeks out, and it's become crisis mode. In this moment, residents should be thinking of others, checking on family, and trying to keep the semblance of community, not looking for free stuff. And in any event, it's a Dollar Store. To further give concession, if it were Home Depot, I'd be more inclined to stomach it, because it may be for repairs and/or building makeshift shelters. But it's a Dollar Store. I'm not really sure where cleaning supplies, a box of Swiss Rolls and a boogie board would classify as an immediate need.