But if you let anybody use your library, and you don't own the copyright to the content, why are you allowed to stop people from snapping photos of the books an

But if you let anybody use your library, and you don't own the copyright to the content, why are you allowed to stop people from snapping photos of the books and posting them on the web? It's one thing if the copyright holders asked you to, but in this case they want as much distribution as they can get.