You may not have written all the stories, but if you’re the moderator of an archive, you’ve accepted responsibility for keeping them online and accessible. And if you’re the publisher of a zine, you’ve accepted responsibility for making the works accessible. We think that transferring an archive onto AO3 is along the same lines as changing web host or ISP or installing an updated software package for the archive; it’s like a librarian moving a collection.
We can also organize an import that requires each creator’s explicit permission before we import their works. This is most common with mailing lists and some zines.