And just how are they supposed to know that the documents are phony? The bearer is nervous, perhaps, or "sneaky looking"?
A registration system with an on-line database for employer reference would remove all doubt about the legal status of an individual with a certain name, birthdate and place of origin, but how do you know the person in front of you is the same one in the papers?.
We can't require small business owners to get fingerprint or retina scanners, digital cameras with facial recognition software or DNA testing equipment.
And a chip implant in the legal workers would really raise a stink!