There can also be a backlash
In 2004, the US donated 40% of all disaster relief funding. That's hardly stingly and it doesn't even begin to count private charitable donations. Pissing people off isn't exactly a smart way to win them over.
Given the UN's credibility problems, especially related to the massive corruption of the "Oil for Food" scandal, some people aren't too trusting to donate more money for corrupt UN bureaucrats to pilfer. Personally, I wouldn't give the UN the loose change that I have in my pocket (all 16 cents of it) but I did donate $250 to the Red Cross today.
