Thanks for the thoughtful response. I'm aware that free distribution of songs is copyright infringement, but this hypo is a bit different and could still fall under fair use even if not a parody or critique. I discussed the fact that this website doesn't charge a fee because "effect on the market" is one of the components (if not the most important component) of the fair use defense to copyright infringement (fair use is a four factor balancing test, see Harper & Row, Publishers, Inc. v. Nation Enterprises, 471 U.S. 539 (1985)). For instance, if we assume that the market of online Diplomacy variant players consists entirely of people who play for free, then you could make an argument that Blizzard (or whoever the copyright holder happens to be) isn't financially harmed by this kind of copyright infringement because "Diplomacy variants" is not a market that Blizzard could enter, develop and have an expectation of making a profit. However, the counterarguments would be: 1. Blizzard's financial interest in "Diplomacy variants' could still be expressed through licensing their ideas, artwork, etc., to variant creators, or 2. A free online Diplomacy variant based on WoW is essentially a substitute for World of Warcraft. Or, if not a perfect substitute, at least that the variant's online presence would have the effect of getting subscribers to play the variant instead of WoW. To me, the first counterargument seems more plausible, the second seems like more of a stretch, but the point is that proof of direct financial harm to the copyright holder is going to be harder if the derivative work isn't making money.