Because my (imaginary) 12 year old son can grab my phone and then pretend to be an adult when doing booze shopping. This is the reason why most Western countries don't allow this kind of technology for age verification. For example in the UK you have to file an application if your new technology can be authorised to be used for age verification in pubs, restaurants and other venues and that wouldn't pass their requirements.

On the other hand, a palm cannot be given to a 12 year old person. If Amazon One uses an element of palm veins in their biometric template, then even if you chopped off your hand it wouldn't work anymore because the blood flow would stop and therefore fail the identification.

It is true that an adult could still pay for booze for a 12 year old by presenting their palm, but that is already the case today where parents can go shopping with their children and buy alcohol as part of the family basket.

EDIT:

At the end of the day, a biometric will ensure that the adult is actually physically present at the time of purchase, which is what the law requires for things like alcohol shopping. Devices don't have that extra layer of quality assurance.

EDIT 2:

> But why couldn't the device handle your biometric authentication? Your 12-year-old son can't (hopefully) steal your face or your fingerprint

Because what happens on the device is unknown to the thing which needs to know that you are of legal age. At the end of the day it's just a phone which says "yes person is above 18" but how do you know that my phone didn't just reply that because it was programmed by me to do so? If the biometric is verified by the Amazon One device which is not my personal device then it's infinitely less likely to have bene tampered with.