The Heritage Foundation is right that it doesn't say much good about the American tax system if half of the population doesn't pay federal income taxes. But I fail to see how that in any way invalidates the point about the unreasonably low taxes paid by the super-rich.

Other than that, the article doesn't mention an important point I learned about one of the previous times I was discussing the topic, which is the payroll tax / FICA. They do pay that, and Wikipedia informs me that for three quarters of Americans (i.e., including the lowest-earning half of that half that does pay income taxes), their payroll tax is higher than their income tax. So it's all a matter of how you look at it - if you include the payroll tax in your definition of "federal income taxes", considering that it is a federal tax levied on one's income, that fifty percent (excepting the retired and unemployed) does in fact pay federal income taxes.