I don't know how it is with the legal crap but considering all games are property of Bethesda I think they should just have most things included in the game as files for the GECK specifically.
I can't mod really so I don't have any use of mods really but still, if most things we inclued as modding files specifically it would be easier for people to mod around with stuff. (I guess.)
I know it might be about that they are individual games and that they should have their unique content, but it could ease some modding parts.
I have no idea if this is in but imagine if Oblivions plants and trees were in FO3's game files for the GECK.
Then someone could have created a vegetated wasteland without using Oasis models and textures.
So I really think that a lot of game files should be included from each game so that the latest game has it easier to mod stuff.

And uhm, yeah as long as you don't redistribute it, have the games and keep the mod personal and to yourself then it's legal. /helpful