I hear Senator Kyl of Arizona explain this. If I remember right, he said it is impossible to enforce this because it is illegal to hire illegal aliens, but it is illegal for emplyers to check to see if their papers are false or not. A true catch 22. I am not against immigration reform, but I am so cynical of government, that what ever law they pass, will never be enforced. The whole issue will never change.
Juan Kyl is just making more excuses. Every employer in this country must obtain documents verifying immigration status on employees as it is. If they do withholding for taxes (which many criminal employers of illegals don't bother with, because it is just a pesky business expense anyway), then the IRS actually does part of the verification job for them because the IRS will tell employers if an SSN turns up fake.