I never buy ebooks from amazon because they come with drm (did once, it was annoying as hell to get it removed with downloading random python scripts and having to start a windows vm, never again). It obviously can't be to stop people copying, because all the books are available for free anyway, maybe publishers stupidly ask for it, but the most likely reason is that amazon wants you to read from their devices so they can show you ads for what you haven't bought yet.