Pretty much every American does pay into the system. My kids pay sales tax. It isn't much - they make $10 a week in allowance (+$5 into a long term savings account) - but their spend is completely on taxable goods - so around here - that means they are paying a 6.5% tax.
Flat taxes are very regressive - at least as proposed. Flat taxes say that everyone should pay - well, lets say 20% of their income in taxes. And that there should be no deductions. Not for mortgage, not for kids, not for medical expenses, not for your 401k (at least in theory, in practice, these will all creep back in eventually making the tax code as complicated as it is today, just without the income bands - which are the easy and, IMHO fairest part of the code). They don't get rid of local taxes or social security taxes - which tend to be regressive. So we replace the one progressive tax we have - income tax - with a flat tax and leave the regressive taxes in place. Can a family making $40,000 a year afford $8k a year in federal income tax? Plus they get rid of a tool the government has to encourage behavior - credits for replacing energy hogging appliances, or credits for buying a more fuel efficient car.
