This is a case ruling which addresses several specific issues of creating a work for use with a specific game. The full case study is available elsewhere or on this site.
Robert Allen creates educational games and sets up a tournament circuit in the schools with his friends. His former friends (now AGLOA, aka Academic games) break with Allen and start their own top-level tournament. They buy Allen's games and sell them to the students, along with a rulebook that references the rules in Allen's games and even reproduce a handful of the rules. Allen claims copyright infringement because his friends are stepping into his market. The court concludes essentially "They're buying your games, they aren't performing your work in public, and their rulebook is not a derivative work because it reproduces ideas, not text." This was upheld upon appeal.
This ruling was made by the Ninth Circuit, and therefore is not binding precedent outside of that Circuit (basically the Pacific coast states). It may be considered as persuasive precedent in other federal cases, but they are not obligated to follow this rule. The Ninth Circuit is considered to be anomalous with respect to intellectual property issues (and a lot of other things), and therefore other Circuit Courts may be less than impressed with their reasoning.
The four games involved in this case were academic rather than mass-market: "A Man Called Mr. President", "Euro-Card" (or "World Card"), "Linguishtik", and "Propaganda". While none of the legal arguments rest on the educational / non-profit nature of Academic Games, it is possible that this weighed in their favor.