Many game EULAs explicitly disallow paid mods, but I haven't seen any specifically mention broader licensing. Even without that, however, mods are a grey area

Many game EULAs explicitly disallow paid mods, but I haven't seen any specifically mention broader licensing. Even without that, however, mods are a grey area in copyright law. Publishers tend to let mods exist because they bring publicity to the game, but when modders start making money, they usually want a cut at best or sue and shut them down at worst.