It wasn't just Kindle. There were dozens of really high quality book reading apps that either sold old or hard to find books, comics, or even mangas. All of them evaporated over night once Apple forced the 30% cut. I'm still very bitter about that move.

Apple even went as far as making some Steve Jobsian rule that you had to sell your book for the same price on the web as you did in-app. So you couldn't make up for the 30% cut. That was a pure bad-faith move and Apple only recently rescinded that requirement.