It's my understanding that bounty hunters in the USA are solely limited to running down bond jumpers. If you have evidence to the contrary, I'd love to see it.
Nowhere did I say that bounty hunters have the authority to pick up illegals. I do think if we're serious about having borders we should consider enlisting them in the effort. States can give them the authority to apprehend and they'd do an excellent job, since many of them come from the ranks of police or military. With the proper authority and some financial incentive the private sector could move a lot faster and more efficiently on this issue than any government agency.