It's not DRM that prevents you from owning the book. It's the licence. The DRM just makes it easier for the licensor to enforce the terms of the licence.
I 'buy' DRM-free books from O'Reilly. The lack of DRM means that O'Reilly won't ever be able to remove my ability to read those books on a new device. However, I still don't own the books, and have no second-sale rights.
