Good question. I would guess four things that make a meaningful difference are 1) libraries are attempting to adhere to first sale doctrine, and attempting to have the ebook “returned” before lending it out again; 2) libraries aren’t trying to use Fair Use as their justification for ebook lending; 3) libraries aren’t scanning books, they’re only offering existing ebooks; and 4) libraries are publicly funded and not a private, for-profit company. Combined, I would guess these things put library lending on a stronger legal foundation.