If you bought the books, you agreed [in the eyes of the law] to this kind of eventuality..
I'm as guilty of this kind of thing as well. We tend to assume an imaginary T&C that conforms to we think is fair when we buy things online - and we have to learn the hard way that the real T&C's are very much more dubious.
Here's the relevant bit in the T&C's:
"Upon your download of Digital 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 Digital Content an unlimited number of times"
"Your rights under this Agreement will automatically terminate if you fail to comply with any term of this Agreement."
The key points is that this is a "license agreement" that grants you rights on the payment of applicable fees.
TL;DR: You don't own any content you licensed to read on your kindle.