Whoa
, this is not true at all.
“Publicly available” and “public domain” are completely different things legally.
Posting a story on a website does not put it in the public domain. Copyright attaches automatically the moment a work is fixed in tangible form, the instant it’s written down, even in a draft. No registration and no publication required.
An author who posts an original story on their blog, on Royal Road, on AO3, wherever, holds full copyright in it, including the exclusive right to prepare derivative works. Continuations, sequels, and stories using their characters and setting are derivative works, and the author absolutely has the legal right to stop others from publishing them.
“Public domain” is a specific status. It can occur if the copyright is expired, the author deliberately dedicated it as such, or it was never copyrightable in the first place.
The legality of fan fiction is often misunderstood: continuations of copyrighted works exist all over the internet not because writers have a right to make them, but because rightsholders mostly tolerate noncommercial fanwork. Legally, most of it is unauthorized derivative work that survives on forbearance.
As an example of that, the 50 Shades of Gray was originally fan fiction for Twilight. It could NOT be published as it was, because the author of Twilight could block it at any time. So it was rewritten as its own thing that made no reference to Twilight characters and settings. That’s the only way it could legally be published.
I would advise making sure we don’t veer into directions that expose us legally. Sure, the law is only as good as its enforcement goes, and I don’t see an author deciding to sue us because we published a derivative work without their permission.
But the law does show that public things don’t belong to the public, they’re just made available to them. That’s two different things.
Someone created a continuation of my The Game story a long time ago. I didn’t like it at all, it violated my lore. I never thought of suing them for it, but I know I would have had the right to. It would just have been a very stupid move. I’m not mad it exists, just, I have my own style and I don’t think it’s easily reproducible. I prefer my stories to remain my stories.