When does derivative-work copyright exist?
For copyright protection to attach to a later, allegedly derivative work, it must display some originality of its own. It cannot be a rote, uncreative variation on the earlier, underlying work. The latter work must contain sufficient new expression, over and above that embodied in the earlier work for the latter work to satisfy copyright law’s requirement of originality.
Examples of derivative works under U.S. law
The most famous derivative work in the world has been said to be L.H.O.O.Q.,[18] also known as the Mona Lisa With a Moustache. Generations of US copyright law professors — since at least the 1950s — have used it as a paradigmatic example. Marcel Duchamp created the work by adding, among other things, a moustache, goatee, and the caption L.H.O.O.Q. (meaning “she has a hot tail”) to Leonardo’s iconic work. These few, seemingly insubstantial additions were highly transformative because they incensed contemporary French bourgeoisie,[19] by mocking their cult of “Jocondisme,”[20] at that time said to be “practically a secular religion of the French bourgeoisie and an important part of their self image.” Duchamp’s defacement of their icon was considered “a major stroke of epater le bourgeois." Thus, it has been said that the “transformation of a cult icon into an object of ridicule by adding a small quantum of additional material can readily be deemed preparation of a derivative work.”[21
http://en.wikipedia.org/wiki/Derivative_work
To a certain extent the legality is handled on a case-by-case basis, however there are some guidelines. This concept of “transformative” works is the most legally relevant. If a derivative work adds something that was not otherwise presented in the original, it generally qualifies as derivative.
http://en.wikipedia.org/wiki/Transformativeness
So, while not all mods would qualify, many would. For example, a house mod, which merely arranges existing game objects in a new and unique way, would likely NOT qualify for copyright protection under derivative works. A house mod which contained extensive scripting language adding functionality not present in the original “game” or copyright material, would be an example of a case that MAY qualify, but would likey require adjudication.
HOWEVER, mods which add new spoken dialogue, entire questlines, written material, custom models, or any other content which was entirely original, would most certainly qualify under present US copyright law.
Therefore, if the mods in question being pilfered by FilePlanet contained such transformative derivative works, AND the modder specifies they have reserved their legal copyright under law for that material, in that scenario, FilePlanet is in violation of US intellectual property piracy law.