Presented without comment: A tweet thread noting that Mojang considers mods derivative works. A rather lively thread on the subject on Minecraft forums. For the record, it's very difficult to argue from a legal sense that any mod built around Forge is not a derivative work of Minecraft. The problem lies in the fact that Mojang doesn't need to you agree it's a derivative work, they can claim that your content is in violation of their EULA if you don't agree that it is, and therefor shut your mod down. The amount of time, resources and goodwill that would be expended legally fighting that makes it unlikely to be overturned.