Mystery of the Expiring Fingerprints

The WSJ has a classic third-column piece on the front page of today’s paper. It is on the bizarro technology-denying behavior of U.S. Citizenship and Immigration Services boffins who, for reasons lost in the yellow-papered past, insist that fingerprints expire, even when they are stored on hard-drives.
Case in point: When I did my U.S. green card a while back I had to be fingerprinted three times — the immigration people kept sending me notices saying that my prior fingerprints had expired. When I pointed out that fingerprints are not like milk, they don’t have “Best By…” dates, the staff at said centers looked at me like I was a nutter. Anyway:

In [the typical time it takes to get a green card], fingerprints expire twice. A set lasts just 15 months — that is the rule.
An applicant is first fingerprinted after qualifying for a green card, which itself can take years. If it then takes more than 15 months for the immigration service to complete the paperwork and issue the card, the applicant is fingerprinted once more.
Those who go on to apply for citizenship are fingerprinted again. People seeking asylum often wait for at least a decade; every 15 months, they are fingerprinted.
The 15-month rule has been around for years. Not even Mr. Yates at the immigration service can explain it.
“It happened so long ago,” he says. “There’s no technical reason for it.”