10-15%? Think more along the lines of 50%. You have to add ~15% in taxes but you don't just have to deal with SSI and taxes. You now also have to deal with accountants and lawyers to make sure everything is on the up-and-up, make sure your workplace conforms to OSHA and state standards, disability claims and benefits, vacation benefits and other employees to get coverage, FMLA, a variety of insurances to protect you from litigating employees, medical benefits...
Not saying that workers should go without all those benefits but for some that is a burden too heavy to carry if the competition doesn't follow the same rules.