From the Kindle Store TOS:
> Upon your download or access of Kindle Content and payment of any applicable fees (including applicable taxes), the Content Provider grants you a non-exclusive right to view, use, and display such Kindle Content an unlimited number of times (for Subscription Content, only as long as you remain an active member of the underlying membership or subscription program), solely through a Kindle Application or as otherwise permitted as part of the Service, solely on the number of Supported Devices specified in the Kindle Store, and solely for your personal, non-commercial use. Kindle Content is licensed, not sold, to you by the Content Provider.
It's clickwrapped, of course. No doubt the argument would be that, whether you read this or not, you agreed to it as a legally binding contract when you "bought" a Kindle book.
I'd like to see it really tested in court, but I don't expect to. For that to happen would probably take a very high incidence of people's accounts and/or access to paid content being revoked for no reason, and as far as I know that doesn't happen anywhere near often enough to make it an issue under public consideration - not that it's other than wrong either way, but I would be surprised to see Amazon legal permit a tactical blunder that would risk the company being hauled into court and forced to behave as if they thought their customers deserved fair consideration.