I would hope they are!

Would be terrible if it was in effect a big user made mod would it not?
I don't understand - I thought that was
exactly what it is. I mean, that's what DLCs and expansions are, and effectively the game itself. The programmers make the GECK and then the designers build the game. What the designers build the game with is - to my knowledge - the same tool that gets released to the modding community, albeit a couple of functions are locked off for legal reasons (the havok/speedtree stuff). The rest of it, though, that's what we have.
If you look at the difference between a fan-made total conversion or semi-TC for Morrowind or Oblivion, they are functionally the same as the professional expansions - the sole difference being that the professional product is more polished. Both fan-made and professional expansions add new play-areas, new models, new textures, new functions, etc.
You could describe Shivering Isles as a (semi) total conversion for Oblivion. Any modder could have come up with that ... with infinite time, resources, and particular skills that no one modder seems to possess. Certainly the Wizards' Islands and Lokken Mountain mods for Morrowind have reached very high standards of quality and are often regarded as "unofficial expansions" - they have all the ingredients of professional expansions but really just lack that polish and professional touch that a development studio can bring. It does prove, though, that anyone can come very close to doing what the studios do, armed with just the publicly-released construction sets. After all, White Wolf of Lokken Mountain was made by only two amateurs in their spare time.
Technically, you could call New Vegas a total conversion mod for Fallout 3. Any of you could make a brand new game using the GECK. Obsidian would just be expected to be better at it than any of us because they've got a lot of experience at this game-making lark.