They do not pay income tax but if they work they pay a high payroll tax. One of the ideas of proposing a national sales tax as offered by Greenspan is to collect some taxes from folks who may not pay much now, to add revenue to the treasury, but to do it in a subtle way by only starting with 3% or so initially. There has never been the thought of eliminating income taxes, at least by people who count. The payroll tax is regressive and that is an issue often discussed by liberals but over the years the liberals have gotten their way and the tax brackets and rates have shifted to offset the regressive payroll tax. I would say a flat income tax would be best but then the working poor couldn't afford the 30%, the small business individual, before expensing income, would pay more, the the rich entertainers would pay less. As it has been said, be careful, you may get what you ask for.
... which they get back as a tax credit (negative income tax) via the EITC.
However there was an exemption of around $30,000 in the original flat-tax proposals. The idea was a 20% (17% actually, pre-Bush pre-GOP Congress) tax on income over $30,000. Personally, I think it was rejected because it was too simple.