The IRS is very picky about what constitutes an independent contractor. This has long been abused by companies trying to skirt employment law and avoid payroll taxes. They have a litmus test of sorts to determine whether someone is an employee or an independent contractor (tl;dr: if you treat them like an employee, they're an employee). And of course, there are significant penalties for getting this classification wrong.
For more details, go to the source: http://www.irs.gov/businesses/small/article/0,,id=99921,00.h...