This is why I try to get drm free ebooks when possible or at least breakable drm. I like paying for it so the author gets paid but i don't want some 3rd party being able to revoke my access.
I've been thinking about this: If this is the only reason you use an ebook-store (like the Kindle store), this would be fairly easy to create a 100% competitor for - after all, there's no law against creating a site for giving money to your favourite authors!
...and then you could create a plugin/API for embedding a "pay for book" button onto other sites - like pirate book sites, and the ONLY weapon Amazon would have against it would be the legal weapon against the pirate sites!
Also you'd probably want the give-authors-money site to be run by a trustworthy nonprofit.
i have seen authors asking people not to do this, because it increases the chance that their publisher will see the book as a failure and not pick up another one of theirs.
Except that the new Kindle DRM is very hard to remove and this plugin won't work anymore - it's a constant arms race where us customers are always the losers.
It's possible, if a little annoying. In your account settings, Amazon offers the ability to download an e-book to transfer via USB. This gets you a version that you can import into Calibre and process with DeDRM.