Few books are sold in an open format
I assume you mean EPUB which is basically just a zipped up subset of HTML, JS and CSS in a particular layout. But it supports DRM and most purchased content is DRM'd. The DRM can differ from one vendor to the next so Apple EPUB files are DRM'd with Fairplay whereas Kobo EPUB files are DRM'd with Adobe Digital Editions.
BTW I'm aware there are some DRM-free EPUB vendors out there selling mostly niche content but they don't represent much of the market and probably never will.
Fortunately Adobe Digital Editions is relatively easy to remove using a Calibre plugin and I think it's prudent that the first thing anyone does with a new book is strip the DRM and back it up somewhere. I don't know if Apple's DRM can be stripped but it shows how even using an "open" format is no guarantee of owning the work or being to read it on unblessed devices.
Amazon's own format used to be a standard called MOBI but has morphed into a proprietary AZW format. There are Calibre DRM removers for that too but I don't know how well they work.
Biting the hand that feeds IT
