They may be or it's possible that they just think that nobody will question them about it. Also, employers who hire people off the books (like day laborers) aren't withholding any Social Security or Medicare taxes either.
"Also, employers who hire people off the books (like day laborers) aren't withholding any Social Security or Medicare taxes either."
Not just day laborers, include most of them working construction.
They pay them in cash and knowing that they are illegals a lot of them make the deductions from the cash wages and pocket the money.
The big savings to the tree trimers and construction is they aren't paying comp insurance which runs 50-100% of wages.