A physical library is either getting their newspapers by asking/paying the newspaper company to deliver them, asking citizens to donate them, or collecting them from already delivered newspapers. If the IA was just piggybacking on user activity (by caching and storing things from a user's browser cache after they visit a page) then I'd have far less of a concern with them. If we're so attached to physical metaphors, this would be equivalent to the librarian running around outside the newspaper's printing room and snatching newspapers from the bundles as the company's employees loaded them onto trucks.

IA is on iffy territory w.r.t. copyright as it is; if they stop respecting robots.txt, they could get into a world of hurt.