You're right, it's hypocritical. But it also says that Buffett a) knows it's futile[*], and b) thinks he can spend the money more wisely than the government. In fact Buffett thinks so little of government's goodness that he'd rather give all his money away than pay tax, as shown by the fact that that is what he is, in fact, doing.
[*What's the net worth of all America's billionaires lumped together, a few hundred billion? We could seize it all and it'd be gone in up the federal government's nose in 4 months or less.]