"It certainly doesn't excuse their employers whose only interested in the cheap labor as close to or below slave wages as possible."
They aren't cheap in wages paid, at least in So. Calif.
From what I understand there isn't one of them that will work for under $10/hr plus transportation and lunch.
As far as construction, they are paid $18-20+/hr. cash. In construction even if the contractor reports his entire gross and deducts none of his cash payroll he comes out way ahead paying the top personal income tax against the added costs of at least 125% over the paid wage on a legal employee.