The "DRM hypothesis" is that the public is dishonest, and will do dishonest things with cultural material if given the chance. DRM is deployed in order to force dishonest customers to behave honestly and buy media and to limit their activities to those that are authorized by rightsholders.Link
For this to work, it must be impossible for a potential customer for media to locate a non-DRM copy of their chosen movies, books, games or music. If a dishonest customer for an ebook can download an un-restricted version of a book that is otherwise available in a restricted DRM format, she surely will.
But DRM is simply not very good at doing this job. Because DRM is based on "security through obscurity" -- that is, in hiding from a user the way that it works -- it is inevitably broken in short order and the materials that it covers are put on the Internet where anyone can download them.
Indeed, there has never been a single piece of DRM-restricted media that can't be downloaded from the Internet today. In more than a decade of extensive use, DRM has never once accomplished its goal.