Well actually the new MMO The Secret World supports TXAA now. So what you mean by dev controlled is that TXAA has to be hardwired into the game for it to work? It won't be able to be forced even with future driver updates?
That is how it was described in the previews. The reason is that TXAA requires help from the code of the game. It must need certain bits of info. FXAA is a post-process AA, much like MLAA that AMD has (though it seems better, but I haven't tried MLAA 2.0), and doesn't require help from the game code.
Edit: I'll be interested in seeing how TXAA is in that game. I wonder if there is any reviews on it yet.