The problem I see here is that fanfic is, in copyright terms, strictly shades of gray but to the detractors and proponents it is either pure black or pure white and neither side pays much attention to the facts that don't line up with their pre-cast position.
Strictly speaking *all* fanfic *is* copyright violation. Letter of the law and all that.
That doesn't mean that nobody should practice it or enjoy it. Just that people should be aware that the practice treads through nebulous teritory, especially when publishing the content publicly.
Also, that when it comes to derivative works, there is a difference between parody, pastiche, and satire (which *can* be published commercially, BTW; C.F., Robert Mayer's SUPER-FOLKS, for one;
http://en.wikipedia.org/wiki/Superfolks or Philip Jose Farmer's various Doc Savage and Tarzan parodies/pastiches) all of which reflect on the original source material at a *clear remove* and fanfic, where the narrative *usually* follows the source closely and with minimal if any remove.
The various shared-universe corporate properties (STAR TREK, STAR WARS, DCUniverse, etc) lend themselves to fanfic because they are by design collaborative efforts intended to accomodate a variety of narrative voices and styles so it is no surprise to see people inspired to try *their* narrative voice and style on the characters that inspire them.
And it should be no surprise that many a writer (though hardly all) might have forged their own voice in those kinds of writing exercises. So there can be be a fair amount of good to come out of fanfic, which is why some authors *graciously* tolerate or at least look the other way at the distribution of such content as long as they don't find the derivative content offensive.
But it needs to be understood that this is a privilege that the authors *allow*, not an entitlement. And certainly no reason to villify an author who does not approve of it, even if the author might be less than gracious in discussing the subject.
Like many a touchy subject, a bit of empathy and manners is necessary if we're to discuss the matter. It's all a matter of degree, not absolutes.