Are you certain they removed themselves?
scAmazon has been noted to suddenly ban and delete content on occasion. And I've heard, sometimes they claw it back from people who paid for it through scAmazon by deleting it from their kindles and anything else they can access.
Read the terms of use at the Kindle store.
"Use of Kindle Content. Kindle Content is licensed, not sold, to you by the Content Provider."
So, it's right there in black and white. You don't "own" it, you bought a license to "access" it.
This is the primary reason I download everything.
More importantly, you bought a license to the version hosted by Amazon at the time you gave them $.
I know this because there are books that an author has updated, but didn't post it as a new book, and I can only get the older version.
Edit:
Something else regarding books being deleted from Amazon.
I know of two ways this can happen.
Plagiarism
Some of the authors who plagiarize work that I've mentioned in various threads here have all their work gone on Amazon (e.g., Leah Larkin, Blake Barkley, Leigh Temple, Millie Carter, & Scarlett Johnson)
Complaints
If people complain about stuff, it can get pulled.