Spin-offs of Pride and Prejudice?

Does anyone else think it's interesting that books continuing the story of Elizabeth and Mr. Darcy, or even their 'daughters' do so well?

The success of Pride and Prejudice has not been lost on contemporary novelists. Practically an entire section of any given bookstore is devoted to spin-off novels based on, or inspired by, P&P.  (www.divinecaroline.com)

I haven't read any of these books, but I've seen many of them on the shelves at bookstores. I've seen this with books about Robin Hood--the Trilogy Hood, Scarlet, and Tuck by Stephen R. Lawhead. However, I understand these because Robin Hood is like King Arthur--there are already various legends about them, a few more aren't going to hurt. What I don't understand exactly, are the many spin-offs of Pride and Prejudice.

I'm not dissing them, I haven't read them, I'm sure they're super romantic and what-not...however, what separates them from Fanfiction? And what genre do they fall under? Is it like...historical fiction? I feel, almost, as if it's hypocrisy for the industry to claim they want something unique when the industry is set up for trends and things that already have a fan base. ahahaha. 

Here's a website of a list of Pride and Prejudice spin-offs.