Well, wouldn't Tanya Grotter be considered a fan-fiction? It even diverges into a completely different plot after the 1st book, where Voldemort wins because Harry Potter dies and the world becomes similar to Orwell's 1984, according to wikipedia: https://en.wikipedia.org/wiki/Tanya_Grotter#Content
So to clarify, I see no good reason why material with no objective value, such as creative works computer data, should not be shared for free. If people like the art they can donate to support the artists. If they don't, at least there's no laws preventing a passionate artist, such as the folks at Kajar Laboratories, from creating.
I don't want to get into a long debate over that specific example, but I believe the courts found that tanya grotter went beyond derivation and into plagiarism.
intellectual property is a legal gray area in many ways. however, it usually takes time/effort/money to create art. to create software and games for us to play, it often takes alot of organization and investment. if they can't profit from their work, then there won't be an industry creating cool stuff.
on the other hand, just because you created something, doesn't mean others can't derive from it, or create works that are influenced by it. sending a cease&desist against crimson echoes was baseless, because crimson echoes is a derived/inspired work, and wasn't even for profit.
but then there's the issue of emulation. you could argue that it undercuts the video game industry. however, it preserves games, makes them accessible, and allows them to be experienced in new ways. alot of emulated games are from older systems/companies that might not be in business. it's also done as a hobby and not for-profit.
so as I said, intellectual property is very much a legal gray area. however, it's safe to say that the artist has a right to control sale/distribution of their work, but not derivative works inspired by it (as long as they qualify as parody and not plagiarism).
there are entire books written about this issue.