Let me preface by saying I'm a big fan of Mr. Buffett and have made a lot of money with him. However...
While it is true that dividend income tax rate is 15%, that occurs after corporate taxes are applied. If a company earns $1M earmarked for dividends and pays 25% corporate tax, the distributed dividend is $750,000. If a 15% dividend tax rate is applied to the dividend, an additional $112,500 in tax is collected. The total tax collected is $362,000, yielding an actual tax rate of 36.25%.
Secondly, as Mr. Buffett almost certainly knows, nothing prevents him and his wealthy friends from paying more tax than they owe. Finally, instead of donating the bulk of his wealth to the charity, Mr. Buffett could put his money where is mouth is and send the money to the government. That he has chosen to donate it to the Gates Foundation, implies that he believes that the Foundation will use the money more wisely the government.