It's actually better in many cases for businesses to hire undocumented workers, since it makes the workers position much more tenuous. They are much less like to unionize, demand higher pay, etc. if the threat of deportation is hanging over their head
Tyson Chicken was shown to have provided written instructions for undocumented workers on staying under the radar. And yet when DHS arrested 900 workers at their facilities, they also announced there was no plans to investigate the company itself (strange - knowingly assisting an undocumented immigrant with such is a felony).