Being interested in the podfic community, I actually see the "author" issue of the fanwork even more clearly. I haven't seen nearly as much "just taking someone else's clips and cutting them together" in the same way I'm almost daily seeing the "you're reading someone else's text and adding a bit of music and sound effects--there is no authorship happening." And yet in response, what you see is a lot of the same arguments as we see fans making vis a vis the original text: we love this so much (show/story); it's a labor of love; there are weeks and months of work in this;....

Not sure that's useful for you, but I think the labor theory may be quite useful...or maybe a possession by time spent with? (I'm thinking of fans feeling like they "own" the text bc they know it more intimately and in more details than the owner, etc)