What do you mean your not allowed? When did they ever say what we could and could not do with mods other than make money off of them?
The Bethesda police will go to your house and take you down!
FO3 and FNV are both Bethesda's IPs, so that stuff about copyrights is bogus. They don't want you to use assets from FO3 (or Oblivion for that matter) because they feel you are "giving the other game away" without the people getting the mod paying for it..
Shortsighted, if you ask me, as mods are what have kept Morrowind, Oblivion, FO3 alive for years, and mods are a great part of the success in sales of FNV.
Myself, I got a whole bunch of stuff from Oblivion in FO3.
Not to mention, FNV has a LOT of FO3 meshes, some straight up, some just retextured, some with a couple polygons added... FNV feels more like a huge mod than a different game, I think.