In short, by deciding to sell his material, Vander Ark was stepping across a line. He was no longer just an enthusiastic fan, but a professional and potential competitor — fair game for the lawyers.
The question now for the courts is whether the lexicon itself violates copyright law, and the decision may not be easy.
U.S. rules allow for the “fair use” of copyrighted material in unauthorized works, but there are limits. Journalists may quote from films and books when writing a review. Scholars can use excerpts from a novel while penning an author’s biography.
your gloss is misleading
Your gloss is misleading. The core issue is not Mr. Ark selling his lexicon for profit per se but rather:
>>Rowling and her legal team acknowledged that readers’ guides like the lexicon are, in fact, allowed under the law, but made the case that Vander Ark simply took too much material.<< and thus: >>U.S. District Judge Robert Patterson Jr. indicated that the case could go either way and encouraged both sides to settle. He suggested that a creative negotiation might produce a book that both sides could live with.<< from the article again, said Rowling: "I never ever once wanted to stop Mr. Vander Ark from doing his own guide. Never ever," she said, before asking the judge again to block it in its current form. What she didn't say is that she's been planning her own Lexicon and wants to block his from publication, to protect her own future profits on her planned official Lexicon...assuming she ever gets around to writing it. She will have to prove in court that Mr. Ark took/quoted too much original material, and as the Judge speculated, that could go either way. I'd say she has an uphill battle, and personally I'd love to see her lose, as a matter of principle, but she probably also has a good deal more financial resources at her disposal than Mr. Ark and may yet outlast him and prevail in a legal war of attrition. Rowling hasn't yet topped the RIAA, but she's getting close in ugliness. Other readers guides of this type have been produced for sale in other genres without controversy or protest from the original authors--what makes Rowling think she's so special? Maybe the court will find in her favor, but I hope for an out-of-court settlement...perhaps a profit-sharing agreement could be worked out--that benefits the FANS above all. Mr. Ark's Lexicon is ready for publication, while Rowling's isn't. If nothing, it's healthy competition which should spur Rowling to simply out-do an amateur like Ark. After all, she's basically the literary "God" of the Potter universe and can make changes to backstories, established literary "canon", etc, that can invalidate and make useless, or at least rapidly outdate Mr. Ark's Lexicon. Moreover, it's clear that Mr. Ark put a lot of hard work as a labor of love building his Lexicon for his fellow fans. I think some financial recompense for his endeavors is not wholly unreasonable, but Rowling seems to think it is. Even if Rowling prevails in court and persuades a jury that Mr. Ark's Lexicon, as it stands, is an infringement of copyright for making too much use of original source material, the victory is a Pyhrric one, for all the ill will and P.R. damage Rowling is doing to herself, not unlike the RIAA suing 13 year olds.