The huji/hukou system actively restricts Chinese workers from both finding jobs or leaving jobs once obtained, or obtaining the pay that job would earn someone registered inside the city/district they work within (for that matter, illegal workers can't go to the government about not being paid--'country bumpkin who doesn't get paid up-front for his job' is an open trope in the PRC and one of the contributors to shoddy work in the usual industries migrant laborers are used in--heavy construction as the stereotypical).
What the US experiences in minor via illegal immigrant labor and businesses skirting rules to pay less for production, the PRC has as a standardized business practice with active government assistance.