Purchased ebooks are like ebooks except you can only read them in Adobe's shitty app, you can't share them with your kids, and you are guaranteed to lose access to them in 10 years or tomorrow. I want to purchase ebooks, but I want to own what I purchase!
Without a DRM mechanism it can be easily shared on warez sites. They really don't want books easily disseminated DRM free on warez sites. Alongside this they can track what pages you read and even know how 'fast' you are at reading. Welcome to 1984.