Need Legal Advice regarding work and Illegal Immigrants?

Hi, first of all, I need to know is it legal for a company(a facility service company) to hire sub contractors knowing EVERY SINGLE person they have hired is an illegal immigrant? Can they do that? They may not be paying those illegal immigrants, but theyre paying someone to hire illegals. Also, what is they KNOW the subcontractor himself IS an illegal immigrant. is any of this legal? I thought you couldnt pay an illegal in the U.S to work. Please help.