Well, okay. There are legitimate reasons to be wary of a move by large, wealthy entities to offer a service like this.
Fan fiction is a discourse; it is one of many ways the meta-community of fandom talks about stuff. It is an ongoing conversation fueled on an economy of attention. The introduction of an economy of actual dollars could, if very successful, disrupt fandom in a way that is bad for people who like the way it currently functions.
Not every—indeed, not even most—fan writers have ambitions of being paid. Those that do can participate in a long and glorious tradition of graduation to the ranks of original authors who were once fan writers, by retooling their fan works or simply employing the experience they’ve gained as a fan writer to create original books. This path has the virtue of being functionally no different from any other path to authorhood—you write a book, a publisher buys it and publishes it. There’s no separate caste of “fan writer” where they get to offer you bad terms and less money. You’re a professional writer, or you aren’t.
The creation of a special sanctioned class of official fanfic could potentially disrupt and homogenize the almost unbelievably vibrant discourse that fan culture currently enjoys—but I don’t think that’s going to happen, because why would anyone eat preprocessed, rules-complient pabulum when Ferran Adria’s making deconstructed masterpieces around the corner and giving them away for free. So there’s that.
However, it could potentially exert a downward pressure on authors who make some or all of their living doing work-for-hire, which is a shitty enough business as it is. Kindle Worlds seems to basically be work-for-hire, minus the cash up front, with really terrible terms. It’s damned near spec work. And spec work is evil.
Finally, it creates a class of sanctioned fanfiction, which seems to imply a class of unsanction fanfiction. And while the OTW is well-equipped to defend the rights of fanfiction writers to keep writing, if it turns out there’s real money to be made here, it’s not hard to imagine a scenario where Disney, say, decides you have to write Avengers fic their way or not at all, and decides to lobby for copyright reform to back this assertion up. I mean, that’s admittedly a pretty dystopic projection, but still.
The metafilter post on the Kindle Worlds nonsense is really good, not that I expected anything less.