
I would suggest that one reason many members of fandom appear to have a vested interest in understanding fandom, and fanfiction in particular, as transgressive, beyond the pale, outlaw is that it serves as preemptive emotional protection.
Fans, especially in the creative parts of fandom, invest a great deal of themselves in their activities. Pure, id-stroking fantasy and carefully tooled craftsmanship are both aired under the title Fanfiction. To have one's work or fantasies or, most of all, work and fantasies derided is painful. To hold up that work-and-fantasy as viable, vital, respect-worthy opens one up to derision, particularly from those who dislike the creative framework one is using.
To proclaim that it is outside the bounds, on the other hand, offers dual protection. Accusations of frivolity or wrongness cannot take one by surprise, since one has already agreed to them. And, by appealing to the Western ethos of the Misunderstood Outsider or Rebel, one can dismiss all detractors as philistines who simply don't understand Art.
I can see the attraction. Unfortunately, as far as powerful defensive arguments go, this one is about on a par with arguing that homosexuals should have civil rights because they're born like that and can't help it, rather than because they deserve to have civil rights.
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