The situation is more complex that that. Although there is general agreement that it is against the law to download complete works that are under copyright, derivative works based upon somebody else's IP are a legal grey area - look at some of the legal battles that have taken place in the past decade over fan fiction. In cases where the owner of the original IP has released their creations under an "open" license or has granted fans rights to use it for non-commercial purposes, you would think that the situation would be clear. But increasingly file locker services are cracking down on these kinds of derivative works because it is impractical for them to verify that the derivative material has been licensed correctly, and they are worried that they may be liable if they get it wrong. The Megaupload case has frightened them and until the situation becomes clearer, they are playing things as safe as possible - hence the growing restrictions on distribution of derivative works that haven't been uploaded through "official" channels.
(As an aside, would it be worth asking Mongoose to provide a "Legend Workshop" area where fans can submit their own creations - making it clear that these aren't official or endorsed by the company...but are legal under the OGL?)
