My immediate thought is Scalzi needs to look at the various laws regarding derivative works. The general rule is, if it is fan fiction, it is a "derivative work" and all rights for any derivative work belong to the original creator of the work from which the fan fiction is derived.
Even if the derivative work contains new, original ideas, characters or settings, all those new features are entirely the property of the original rights holder who the derivative works belong to. Therefore, all the stuff Scalzi talks about regarding the world license owners being allowed to "mine" your ideas from your fan fiction is a moot point - this is their moral, legal and deserved right. If you write fan fiction, and you don't like this aspect, then stop writing fan fiction.
IP holders have always had this right, and have not been given anything unusual or new by Amazon. The difference? Amazon will pay a royalty to the fan fic writers as well as the IP holders, where previously the fan fic writer had precisely zero rights to anything from their derivative works. So in other words, fan fiction writers can claim an undeserved (in a legal sense - I do not wish to enter an argument about the morals of it) royalty for works in which they have no legally defined moral rights of any kind. Even with all the other rights grabbiness of the scheme, the fan fiction writers are getting a better deal than they could legitimately have hoped for.
I am glad I do not write fan fiction, have never written fan fiction, and do not intend to start writing fan fiction. However, I know of some fan fiction writers who would be excited by this deal, because they know full well it is the best deal they were ever going to get. As somebody once said on a site I was reading years ago which had a fanfic section, it is a good deal if you don't get sued.