Actually there are. And it would be great if there was just a checklist that you could go against, but the best we've got is this page [1] from the IRS and some legal interpretation. I agree that the positions could more clearly state that it was a contracting position, but it looks like more of an employer-employee type job board than an eLance type deal to me. So they just have to be careful.
[1] http://www.irs.gov/Businesses/Small-Businesses-%26-Self-Empl...