The US is #1 in charitable giving because it's so easy to get a tax deduction for doing so. If people had to give out of their after-tax income I suspect you'd see a quite different picture.
Can you provide some sort of proof of causation for your theory?
'The taxable income of the donor is reduced by 300 USD. If the donor's income was in the 35% income tax bracket both before and after the deduction, the donor's tax liability (amount of taxes owed to the government) is reduced by 105 USD.'
http://en.wikipedia.org/wiki/Charitable_contribution_deducti...
(The other thing I've wondered about is how much of the US's charitable donations are simply tax fraud).