For some reason it strikes me as a pretty good idea. Upon further reflection, I think it has something to do with:

- Trust -- people increasingly distrust sites like Facebook or Google (privacy concerns), but Amazon still has pretty much entirely "positive" feelings for consumers. And if they can run AWS as well as they can, then you assume you can trust them with your password too

- Micropayments -- your credit card is already linked to your Amazon account, presumably, so it suddenly enables you to pay for content, etc. on a wide range of sites where you might not otherwise, due to friction and trust issues

It's funny... for some undefinable "fuzzy" reason, I feel much more willing to log into a site using Amazon credentials, than I would with Google, Facebook or even Apple.