Actually, that's not quite accurate.
The current immigration law only requires the employer to keep copies of documents showing that the employee is legal.
There is no requirement to "make sure." As a matter of fact, the Social Security Administration and CIS/ICE will not inspect these documents or "make sure" of anything. And since the employers are no experts, there is a loophole big enough to drive through a pickup truck full of illegals.
Going by all the employers for which I've worked, most illegal aliens were hired because they had papers. Fake papers, but the required papers.
And with the advent of the computer age, even the best experts have a hard time distinguishing between fake and real documents.