That's because we confer and infer specific rights in different situations there. Selling something confers ownership, and lending something confers right to use, possibly limited by stipulations at that time. Re-lending something may or may not be allowed depending on what the original lending agreement was. In the metaphor, limiting in this way is like your friend saying "here, but don't let anyone else use it, I trust you, not them" when it's lent. For businesses doing some sort of lending it's generally more complex and a contract (like what you sign when renting a car), and may or may not cover usage by other people.

I have no idea what limitations, if any, are put on the lending of digital goods by law or by the specific terms of IA, or the terms the other libraries in question offer.