I've had the same thing happen to me at the Walmart self-check lines. Apparently, it's incorrectly coded as alcohol, so the system freezes until the attendant checks your age. I've asked them a few times why this is so since anyone can buy no-alcohol beer and the only response I've gotten in reply is "I don't know."
“Non-alcohol beer” is a misnomer. It still has alcohol in it, but only a very tiny amount.