The difference is a matter of scale. I believe the loaning of books is allowed the same way you could loan out a physical book to a friend, etc. Loaning out a digital book to n people is just not the same. Likewise, libraries, which do loan out physical books to n people (but still less than is possible with a digital loan), pay more for their books for this very reason. If you want to loan out your digital book to n people, then you are going to have to pay a lot more for it (and that is just not part of the current equation).