Read other letters about this article
The Google analogy doesn't work.
For starters, Google and other search engines direct people straight to your work. It's why they exist--they're glorified phone books. They are not profiting from your words, they are profiting from directing people to your words. The fact that they have advertisers reimbursing them saves you, the author, from having to do the same--they are providing you as much a service as they are providing your readers.
In the case of the Lexicon, the author profits directly from JKR's work, not from a service he is providing to her work. It may be in some cases that someone picks up the Lexicon and then starts scooping up Potter books. However, it seems more likely to me that purchasers of Potter books would add the Lexicon to their Potter collection, not vice-versa.
Google's copy of Salon does not hurt Salon because Google isn't, for example, choosing the best of Salon's content for the day or week, publishing it, and selling it on news stands. Again, Google isn't using Salon's content for the content's sake, they are using the content to drive people to your site. They exist for no other reason.
In addition, it's not so much about the Lexicon "hurting" JKR, is it? Whether it helps or hurts her, the Lexicon directly takes her work and profits from it. Even if it helps JKR, she should retain reasonable rights to it. Maybe if I went and printed a thousand T-shirts with Harry Potter's mug on it, even more people would be exposed to the books and that would help JKR too. It doesn't make it right though, morally or legally.
Finally, you do point out that Google has an opt-out available. Isn't this what JKR is doing?