But they don't charge you anything to store the book, right? Charging something for the download is presumably necessary to discourage gaming the system and the rate chosen is meant to encourage books they consider the right size which may penalize (intentionally or not) image-laden books.
I'm not arguing that that Amazon's policies are right or that "Unlimited" makes sense for authors (or readers), just that there are good reasons to charge something.
> On AWS, you're charged $0.15/TB of bandwidth out
Where did you get that? Data out starts at $0.09/GB, goes down to $0.05/GB for big customers and presumably can go down even further for even bigger customers but not 300x cheaper.
https://aws.amazon.com/s3/pricing/ https://aws.amazon.com/ec2/pricing/