[Fat Charlie]

Because he's not volunteering to give his own money to a government that's run by people who don't care about doing math properly. That would be horribly inefficient and would reward bad behavior. What he's doing is criticizing a system that lets Warren Buffett pay a lower tax rate than his secretary does: that's a horribly inefficient system that punishes (by charging a higher effective tax rate) working stiffs.

Apparently the authors of our tax code think Warren Buffett is pretty hard done by and shouldn't be taxed as highly as his secretary, his janitor, or the guy who cooked the Big Mac he had for lunch. Warren Buffett thinks pointing out that this is wrong is a much better way to improve matters than simply giving more of his money to the IRS.