I think I have a moral sense that the requirement to ask an author's permission before podcasting or remixing is different from creating fanworks from a professional source without permission.

I agree with this, and I think it's because the source canon is on a different level than any fic, art, etc. that people produce for it. That is, if you write a fic, no matter how much people love it, call it "their new canon" or whatever, the original is still the Truth for that universe. It is inherently more legitimate than the fic.

Conversely, fic does not have that kind of legitimacy over other fic. The original and the remix are on the same level, which can lead to problems fic based on original canons does not encounter. So I think those who believe that remixing a fic is the same as writing fic based on an original canon are really missing the elephant in the room.