If writing is a "craft," and you're expected to exercise your skills by pushing the boundaries, "stretching" the characters into new and different forms, why wouldn't it be just as true in original fic?

I suppose it's because (and I'm totally theorizing here) there already IS a canon Napoleon Solo (for instance) so maybe that means the "art" is in varying extrapolations of him, rather than hewing to a narrow line? Whereas in your own fiction, if you write character A one way in one book, and another way in the sequel, people will go "Sheesh, doesn't the writer even know her own character?"

I'm not sure that's logical or anything ... but of course so many things are OK in fanfic that you couldn't get away with in published work.