Its not easy, actually. Subcontract employees present all sorts of complexities, respondeat superior comes to mind as a glaring one: You send a contract employee out to a jobsite and he blows up your server through negligence, but in the normal course and scope of his duties. You call your liability carrier. They tell you to get laid, because subs aren't covered. They are supposed to carry their own liability coverage to meet that definition of "subcontractor." Your contract employee says, "hey man, you ain't paying me enough to carry a bond and my own liability insurance, so get laid."

And in reality, you would have never gotten the bid for the job anyway, because nobody would hire a farmed out subcontractor that has no liability coverage.