Thanks for the question - not the first time it's been raised. I posed this very question to Chaosium back in 2005, here's a quote:
A keeper who owns a Chaosium book can run material from that book for play-by-mail/post, so long as the game is free to participants.
If the keeper has purchased a scenario book from Chaosium, he or she can --to a point-- do with that physical book as desired. Extensive quotes from copyrighted narrative text are discouraged, unless per normal 'fair use' . Wordages and other restrictions vary from country to country. A hundred words or so is the nominal limit for the United States, for a short story or article.
If reproducing and circulating maps, plans, handouts, and illustrations for the players, each individual item should bear the Chaosium copyright statement and the current year, for example:
_copyright (c) 2005 Chaosium Inc._
Especially for illustrations and complex graphical material, that legend should be superimposed onto the image itself, to guard against further reproduction and to protect the artist. If the keeper were to redraw and supply his or her own graphics, he or she could copyright the resulting item. Extensive quoting of narrative text is cautioned against.
The existence of a forum would not allow keepers to bend or violate copyright laws, no matter the ease or difficulty of electronic transmission. If the keeper follows the rule of 'if I own it, I can present it as a pbm/pbp', he or she will be well within the law.
We seek a good faith effort, and have no interest in screening the solutions keepers come up with for special situations.
Copyright on individual posts on this site resides with the author - however this is a moderated forum posts can be deleted. There are some technical 'grey areas' about playing in games and copyright, mostly to do with 'original characters' (and that's my opinion not legal fact) - however as stated above
We seek a good faith effort.
The addition of a 'fan-fiction' section doesn't necessarily complicate matters. The poster of 'original' works has copyright (totally and unequivocally) - however the act of posting here - would grant us (this site) publication rights.... namely one time publication on this website only. This will be made clear when things are officially launched.
The actual mechanics of the new Fan Fiction section are still fluid and will be subject to change (mainly presentation) for a while.
I think it's prudent to make some copyright statement where we can all see it.
Hope this helps?