not entirely serious, but not entirely joking: in a world where all stores are like this; you drop your phone and it smashes, can you still buy food?
(what about people without phones?)
(what about people without phones?)
In a world where services that require phones are dominant, choice narrows for elderly and low income persons.
[edit] I don't think that's how Amazon implements it at the moment; it just doesn't seem like there's an inherent limitation in the technology that would require customers to have phones. It's just that it's only open to Amazon employees at the moment, so pretty much every "customer" is guaranteed to have a phone.
Not having a phone is definitely a big worry. Not all people even have a smart phone, even in the US, and of the people that do a lot don't have credit cards. One feature we're working on is a fully anonymous and phone-free checkout process if you don't have a phone. You walk up to a kiosk that tells you what you have and then you just insert your cash.