Posting something on the public internet is not consent for you to scrape it and post it on your own site forever.

And requiring an explicit opt-in would basically mean no IA.

To be clear, the IA is a positive, maybe even a great one. But it skirts by because most people don't care. (They did as you say post whatever on the public internet.) Add the facts that they're a non-profit, aren't trying to monetize their hosting, and will generally take things if the owner asks.

Libraries and other archives have some very limited special rights (which mostly relate to making physical backups of physical books). But invoking "library" isn't some general get out of jail free card with respect to copyright.