How it works, is you can request a story to be written. If an author chooses to act upon it, is up to the individual author. Same goes for a writing challenge. Where the problem is, is charging and paying for fanfiction. We, and many other sites, get by with hosting fanfiction because it's free, and no money is exchanged for the writing of it, or the publication. If there was money involved, you can bet that at some point, there'd be infringement problems.