There is also the possibility that they hire subs with immigrant workers.
A lot of times those foreign crews even if the whole crew is legit are willing to work for 7-8 dollars an hour.
So this company might wind up subbing to companies who are fully bonded and insured etc, and who just have really low paid workers, and then the company takes their percentage off the top.
The other possibility is that they advertise for fully legit subcontractors, and they get contractors who have all of the necessary licensing, but who pay their immigrant labor under the table at extremely low rates.
The company subbing out wont care either way because they are covered, so they can make their profit that way, and still afford to bid competatively with other companies.
