I think that Fan-fiction is a great way of someone writing out their own fantasy of the famous thing. If the critics say that fan-fiction is illegal, many people, mainly teens, will revolt against them, and it will be a mess.
Ugh, I don't like fan-fiction at all. Sure, I've tried to write it before, but it's never as great as the original. It's basically like making sequels of movies by different directors. It's not original to the storylines set out, people usually don't like it, and it takes away from truly novel ideas.
I think you're confusing fanfiction with for-profit fiction inspired by/based on other texts. Fanfiction is amateurist by its definition, it's something fans put together because of their love for something (an author, a text, a game, a film, etc) not because of their love for money that can be got from people who love something - which is what drives the publication of DW, Star Wars, Buffy, etc media as well as modern adaptations of older books.
It's interesting that critics have even acknowledged fan fiction at all. I thought it'd forever be an underground thing and pretended like it never existed.