Isn't that a little cheap?
That's the idea.
Mods are free, bethesda doesn't directly benefit in the name of cash for them, but does benefit in the name of an extended lifespan for the game. Last year I went out and bought Morrowind from Best Buy. In case you're not aware it has been around since before Oblivion came out. That sounds to me like a pretty long lifespan which most likely wouldn't have been possible without mods.