Self-employed persons have to pay taxes on the first $106,800 of combined wages, tips, and net earnings (subject to a minimum of a mere $400). Only half of that amount can be claimed as an exemption on gross income, which means that a self-employed person is effectively penalized for not working for someone else, especially those with low incomes. For 2011, the rate is 13.3%, which is an enormous amount at the lower end of the income scale.
There is no legitimate justification for maintaing this tax as it is presently. It effectively helps corporations at the expense of small businesses and individuals.