Originally Posted by Loren Pechtel

Actually, there is a good reason to fingerprint non-citizens: Espionage.

When presented with a US passport they can check to see that the photo on file matches the person and that the passport is valid. It would be pretty hard to slip in on a forged/tampered US passport.

They can't make such checks on foreign passports. Fingerprints won't catch forged foreign passports *THE FIRST TIME* but it means that once a spy has come into the country once they can never come in under another identity.