ao3 has protection against basic data scraping which is used to train AI, taking works offsite exposes them to that risk also, a lot of these fics may have been only accessible for registered users on ao3 and sharing it is going against the author's express wish that the work only be read on ao3
This is an extremely good point. Nothing wrong with uploading your own fics or sharing things privately amongst friends. But don’t reupload work that isn’t yours publicly. Even with the best intentions, it’s not yours to do whatever with. #ao3