@syntax It is annoying. The answer is money. They make more money by renting and ensuring you cannot share your books with friends as well as getting revenue from intelligence collectors. A few stores such as https://www.orbooks.com/ allow you to buy outright without DRM rather than rent. It is often possible to remove the DRM from EPUB's with something like Calibre, but it is a hassle. I recently made https://www.ebooks.com/ refund my purchase after finding I had to use their spying app.
@krock Yep, I have Calibre and think it's a great tool. For most of my years being a serious reader I've preferred printed books, but I like the idea of getting back into ebooks. I have an old Kindle Paperwhite (which I keep off all networks and on airplane mode so it can't phone home to Amazon). Soon I might put some time aside to work on a personal ebook library (with the help of Calibre).