There have also been cases where a customer violated Amazon's terms of service and lost access to all of their Kindle e-books. Amazon has all the power in this relationship. They can and do change the rules on us lowly peasants from time to time.
Note, there are indeed ways to download your books and import them into something like Calibre (and remove the DRM from the books). If you do some web searches (and/or search YouTube) you can probably figure it out.
You don't own your Kindle books because you bought them from Amazon
I don't own mine because I pirated them
We are not the same
edit: I actually try to circle back around and buy physical copies of any book I really enjoy. But I'm much better about paying for video games, tabletop games, and even journalism than I am fiction... I think my bezos resentment gets in the way a bit there.