I don't really get how a DRM free library works. Based on the article at least it seems like being DRM-free means people can keep the book as long as they want? What's the difference between that, and just letting anyone download the book for free online?

DRM for books will of course always be easily bypassible - it's just text and images, not exactly hard to copy. But I don't see how a "library" can work without some limit on how long you can be using a piece of content.