Ezra Klein argues that they don't:
When you look at percentage of total tax liabilities, the rich do in fact bear a heavier burden. But it's because they have so much more money. They are not bearing a heavier burden as a percentage of their incomes. They're bearing it in relation to everyone else's incomes. Indeed, it's only because the sheer levels of income inequality in this country are frankly unintuitive that Fleischer can even write this sort of dreck. People hear that the top 20 percent pay almost 70 percent of the country's income taxes and nod their head. That's unfair! But it mainly seems unfair because people don't know the top 20 percent accounts for almost 60 percent of the national income.