There is a lot that government does that is counterproductive and/or wrong, but national defense and law and order, which are legitimate governmental functions, are also provided for through tax dollars. The American taxpayer paid for the stable and safe world where he made his millions and even now the suffering Americans are providing for the bulk of the safety he enjoys in Singapore. Freedom isn't free. The citizen who pays his taxes is doing his patriotic duty in providing for the common defense.
Fine. He paid his taxes while he was a citizen, and will still pay the exit tax ALREADY in place from the '90s. And now he is no longer a citizen of the US. That's his choice and the US should have no additional claim on his wealth.
Also, it is not patriotic to pay any more tax than the minimum under the law for any citizen. No more is required than that.