best and most complete answer....it's not really income tax....it's the IRS collecting social security and medicare taxes.  Only the poorest of the poor self employed can escape being taxed somehow (ie, <$400)

when Self-employed you can't escape the SE tax of 15 %.

better answer was "you're paying some medicare and social security taxes, really....not income tax" on ANY money over $400 in Self Employed income.