Actually ... you've got an interesting point, because you're right. I've read 1st-person original-character POVs that I really enjoyed, and I've also never had a problem with 1st-person fanfic in fandoms where the source canon is in 1st person (like Dresden Files).

I wonder how much of it is ownership and how much of it is just being used to seeing the characters from the outside, so to speak -- so a 1st-person OC is still the same thing (watching the characters from the outside). I'm not sure why it should make a difference, and yet, it does seem to.