Many, thought not all, employers hire illegals who use false ID and a fake SSN. That was the case with Wal-Mart. They make all those SS, Medicare and FICA paymments not knowing the person they hired is not who they claim to be.
"That was the case with Wal-Mart."
Unfortunately you are incorrect. The employer of the illegals was a subcontractor for Walmart. Walmart was guilty of not checking the I-9's of it's subcontractor's illegal employees.
When not one of your janitors can speak a word of English and they're all "contract" labor who will work for $5 an hour with no overtime, workman's comp, --- then you can make a pretty good guess they're all illegals. Contract labor is one of the biggest scams going --- replace your employees who fall under the labor laws with contract labor and you can get around all the labor laws and taxes --- but Americans tend not to like to collect government handouts when they're working 40-50 hours a week --- so you have to find Mexicans who do.