SaltyNZ:
Actually, I think the real reason is not that he has been underpaying, but that he has legally paid a tax bill that would make the average citizen incandescent with outrage. That is to say, not that he (for example) paid $100M but should have paid $150M, but more that despite being a billionaire, he legally was entitled to only pay $10K due to his financial structure, and did so.
Perhaps one of the reasons why the US is $20B in debt, too many businesses creaming the tax system. It's a global thing though and probably explains why most western countries are seriously in debt. Time for a flat tax rate across the board, no perks etc.