I'll never understand fan fiction as long as I live. ALL fiction to me,is fan fiction, it comes from the author mimicking and being inspired by other works, it cannot exist in a cultural vacuum. Marlowe begat Spenser begat Bosch. Seems pointless wasting hours and hours of writing time on already invented characters, when with a quick name change and some details altered, you have your very own fiction.
Not that it bothers me though. Each to their own and if they're having fun doing it, well who am I to say they shouldn't?