unfortunately, e-verify is voluntary except for Federal contractors, where it is required. Probably will take an act of Congress to make it required across the board ...
https://www.google.com/search?q=e-verify+requirements
E-verify is NOT voluntary if you have a standard payroll and pay payroll taxes. Even small companies are required to validate via their payroll agent (ADP, bank, etc.).
I implement HCM systems and know of what I speak.
Under the table OTOH follow no laws.