Earlier quoted context omitted.
> "why would someone want to log in with Amazon"? > There's no social network attached to your Amazon account I think you answered your own question.
This. I avoid accidentally posting some garbage to my facebook wall. I'll finally start using "Log on with" if I can use Amazon.
Since chargebacks - unlike social spam - cost money, Amazon has every reason to be more cautious about their permissions and confirmation screens, but the wary user probably isn't unjustified in hesitating when asked to sign up for a free service using a login they normally use to buy stuff with.