This is because wit hthe games mods people still require the game to play the mods and therefore the mods are still the property of Bethesda. Posters on the other hand, even when not for profit by the artist, the creators of the content in which the poster resembles is still the property of Bethesda. Therefore they do not want it to be used as the artist would be gaining recognition by using Bethesda property. It all has to do with intellectual property rights which not only includes artwork, music etc etc, it also includes the concept which these posters would be based on.