I know next to nothing about Haiti, so unlike most people on the internet, who I'll concede have spent entire minutes researching the Caribbean, I won't say that much about it. But a couple of things occur to me about our response:
1) I'm more likely to donate when there's an unobtrusive sign at the cash register that notifies you that you can tack on a donation of, say, $1, $5, or $20 to help, than when the cashier asks me to my face in front of a line of onlooking customers. It's an attempt to use social shaming for the greater good -- "I mean, you don't want all these people here to think you're some kind of heartless skinflint, do you?" -- but it backfires. We resent the impropriety of being put on the spot like that.
It's like if some fat guy is buying two pints of ice cream, a pie, and some candy bars, and the cashier asks him earnestly, "You know, there's probably several pounds of sugar in all this, and that will only further harm your health -- are you sure you want to buy this?"
For some reason, we don't resent it so much if the attempt at shaming came from the other customers in line, like "So, are you going to donate?" or "Jeez, that looks like a lot of sugar." I think we feel like they are our equals, and so we accept shaming from them. We're all trying to help each other act better, and today it just so happens to be our turn in the hot seat -- "OK, you got me." The cashier, though, we perceive as an authority figure -- only they are able to put a given customer under that much pressure, they're the ones taking our money or making us sign forms, etc. -- and so shaming from them strikes us more like an abuse of their power. They're just putting us in the hot seat to signal that they can, and that we can't do it back to them, we think.
2) There are already a couple of references around the web to Adam Smith's "earthquake in China" passage from his Theory of Moral Sentiments. Basically, the quoted part says that a person will lose more sleep over the dread of losing their own little finger than they will over the knowledge of the deaths of millions of far-away people swallowed by an earthquake. Surely that's how most Americans are responding to the news of the earthquake in Haiti.
It's the only part of the paragraph that is ever quoted (see WikiQuote's entry on Smith, for example) because most people who like quotations are either cynics or pessimists. You rarely go looking for a good quotation when you're feeling sanguine. By only quoting that one section, the cynics get a rush from implying that human beings claim to care about others but really only care about themselves, and under the same reading the pessimists are relieved to uncover another aspect of human nature to bemoan.
But read the rest of the paragraph here (number 46, or ctrl F "china"). He adds that, although we appear to value our own little finger more than millions of Chinese in the previous example, what if we could push a button that would save our finger but also kill millions of Chinese? Suddenly we have the exact opposite reaction -- we wouldn't dare value our finger more than all those other people. He's drawing a distinction between passive and active moral behavior. We appear more callous when we're in passive mode, but once in active mode we behave much more morally. That split shows up a lot in the psychology lit on intuitive moral judgments.
So here the cynics and pessimists are wrong: when put to a real test, most people accept that they're no more special than others and will take a hit rather than cause harm to innocent others in order to save themselves.