One thing to remember here is that the flip side of "no one knows whether it's under copyright" is "no one knows whether it's public domain." While public domain material certainly has a creator, it is legal to distribute it, create derivative works from it, and so on and so forth. I can direct a Shakespeare play, or turn it into a comic book, or record a spoken-word version of it, or just publish it as a book, even though I'm not Shakespeare, because his work is in the public domain.

The problem with works in the "no one knows" state is that some of them are in the public domain at this point -- but no one knows which ones. A good orphaned-works policy should protect creators who still hold a valid copyright by making it easy for them to enforce that copyright if they discover that their copyrighted work is being used, but it should also protect users who really do only intend to redistribute a work or create a derivative work from it if that work is actually in the public domain.