Furthermore, we HAVE to do it. That's something no one addressed, in those thousand comments about Diana's blog. There was a lot of talk about copyright, and whether or not fan fiction was illegal, whether it was fair use [...] but no one mentioned one crucial aspect of copyright law -- a copyright MUST BE DEFENDED. If someone infringes on your copyright, and you are aware of the infringement, and you do not defend your copyright, the law assumes that you have abandoned it.
I allow that this would be a really strong argument against allowing fanfic if it were true. But guess what? It isn't.
This particular misconception comes up a lot in debates about fanfic. While it is generally true of trademarks -- if you don't defend them and keep people from using them as generic terms, you can lose them; that's what happened to xerox and kleenex and aspirin, for example, and that's why Google fights against the use of "to google" as a lower-cased verb -- it is not at all true of copyright. As of 1978, in the United States, any creative work placed into a fixed form is, at that point, copyrighted to the original creator. This is true regardless of whether the creator asserts copyright on the work, registers the copyright with the Library of Congress, or defends against unauthorized use. That copyright is yours, and nothing save the passage of time (as of now, 70 years after your death) or your decision to sell the copyright to someone else or release it into the public domain can change that.
I can understand why published authors might be queasy about fanfic, and I don't really judge anyone for disallowing it (practical issues with attempting to do so aside). But I don't think it's too much to ask them to base their positions on actual facts.
Edited to add a link to this beautiful defense of fic, a list of what fanfic is and is not that rings perfectly true to me. Strongly recommended.