an ebook that's yours to download is one thing, an ebook that you lose access to once your subscription ends is another. vendors love locking you into a platform and having you "buy" content that's never really yours.
Textbooks are the real killer; soaking of a captive audience that has to buy what the professor/school requires (often after making their own deals with the publishers). It's especially annoying when school adds its own random books to the program.