There are undocumented workers in every industry, getting exploited in a hundred different little ways. Some political factions have done very well out of creating a caste of legal untouchables.
While the USA exploits undocumented workers in agriculture etc., its not like being documented is a huge step up either. In Canada, there's a whole (sub) class of legally imported labour under the "Temporary Foreign Worker" program that's exploited in a similar way.
A big difference is that a person working in the country legally can report labor code violations to the authorities without risking immediate deportation.