Part of the legacy from the years of the regime of the "Former Occupant of the Oval Office, 1993-2001". Several nominees to Cabinet level posts and administrative positions were withdrawn in those years because of a "nanny problem", illegals who were employed and for whom no payments were being made to FICA on a timely basis. This established a precedent, which is another example of political correctness once again carried too far.
"What did this person know, and when did he (or she) know it?"
Once, it was entirely possible that an employer might NOT have known the status of a foreign-born employee, and it was not all that uncommon for a domestic servant to not have Social Security funds withheld from pay and forwarded to the US Treasury, a requirement easily overlooked by someone who casually employed only one or a few people for domestic purposes in a year.
There's more than just the nanny problem. Mark Hosenball with Newsweek was on MSNBC a bit ago via phone and actually laughed (with obvious glee) as he related that he had faxed the WH yesterday afternoon with news that there was an arrest warrant issued for Kerik back in '98 or so.
He said the record was murky from there and they couldn't even establish Kerik had been served----the warrant was for non-payment of dues or some such, btw----but 2 or 3 hours later Kerik withdrew his name.