Anna Gorman offers this article which contains a highly misleading statement:

When federal authorities catch illegal immigrants on the job, some U.S. employers have a ready explanation for how they came to be hired: It wasn't us. It was a contractor.

...For now, work site raids and prosecution are infrequent, except where immigration authorities perceive a national security risk. But even as pressure builds in Congress to crack down on the hiring of undocumented workers, the involvement of contractors and subcontractors could make enforcement nearly impossible, labor experts, government officials and immigration policy critics say.

Under current federal law, companies face criminal or civil sanctions only if they knowingly employ undocumented workers.

"An easy defense … would be to say that they used this subcontractor who they assumed was checking the documents," said Jennifer Silliman, assistant special agent in charge with Immigration and Customs Enforcement in San Diego. "It gives a level of deniability."

So, you go after the contractor then. It's not like they have a special immunity. While many contractors will no doubt be fly-by-night, if enough of them are prosecuted many fewer will want to take the risk. That will encourage contractors to make sure they hire legal workers.