I don't think anyone is toting this as a "diet" but Domino's is trying to provide options for people who might not otherwise be able to eat their food.

As far as I can see, they are TRYING to be responsible in stating clearly that while their pizzas are made gluten free, they can't promise that the ingredients won't come in contact with other gluten-containing items. If anything, they are trying to be MORE responsible.

I'm really not sure what exactly it is you think they've done wrong, but just based on this, I can't find anything that looks suspect.