Why does the us border scan your passpaort when crossing back from mexico to the us?

To verify your citizenship and your identity.

FYI you can cross back and forth with your current valid US driver's license if it has a photo. I went to Juarez last month and didn't need a passport.

They are verifying that it is not a fake.

To make sure your not trying to smuggle mexicans into the U.S..Not to be racist but we have enough already

There are various reasons for the scanning of a foreign nationals passport when entering the U.S. Making sure the passport is valid, not fraudulent,and that there is no watch or warrant outstanding for the person indicated among others.If the bearer is a U.S. citizen it is done as a matter of convenience for the same reasons. However, I have in my life been back and forth across the border (US, Mex) hundreds of times and have never been asked for a passport or any other type of Identification. I do not think that there is any law, unless it is recent, that requires a U.S. citizen to bear a passport to reenter the U.S.coming back in from Mexico BY VEHICLE OR WALKING since there is no requirement from Mexico (for a U.S. citizen) to have a passport to enter. However, you should understand that you COULD BE DETAINED until you did prove your citizenship in the event that you encountered a fussy officer . So while I never was asked for proof in 40 years,( probably due to a midwestern accent), I did always have a drivers license that I COULD have used. A passport is NOT required to cross into the free (tourist) zone of Mexico.