Because their entire argument thus far has basically been “but we’re a library.” But that completely misses the point that even libraries need to comply with licensing laws. Even with ebooks, they can’t just lend an unlimited number of copies. They have licensing agreements with the publishers, to be able to lend [x] copies of [y] book at a time.
They purchase digital licenses to be able to lend those books, and they can only lend as many licenses as they own. Just like physical books. They need to use time-gated DRM to automatically revoke access whenever the rental time is up.
And at first, that’s exactly what IA did. But they decided to disable that DRM, and just start lending unlimited copies to people instead, which flies in the face of established copyright law.