Tensions seem to arise when the fabricator takes the story in a new direction that the previous author doesn't approve. This seems to be the partial case in regards to the what is occurring with the Harry Potter stories. There is an author that wishes to release an encyclopedia of sorts, of Harry Potter. JK Rolling, doesn't like the idea so much because she wants to retain rights to the sole integrity of the Harry Potter "act."
Deciding where ownership lay in regards to intellectual property is a fishy topic. On the one hand you have someone that claims to have created an idea, thought, or character, but how does one prevent that idea from being exploited, and who enforces these laws? In my opinion it seems that ownership should exist in terms of copyright laws. If you have written a book, and worked hard to complete it, shouldn't you be given the rights to what you have accomplished. Furthermore if someone comes after you and chooses to expand upon your work, shouldn't that be their right, as we are all influenced by someone elses work, I mean that's how we end up where we are today.
Fan Fiction is reminiscent and poses the same potential and problems. If we allow fans to take the original piece of work in whichever direction they wish, it is their given right as a human to do so. If we decided to restrict them and declare ownership of ideas, thoughts, characters, and etc. then what direction are we heading in, and who is going to enforce these rules?