Gluten free people can eat the wheat free pasta though, can't they? In which case it's probably not worth the supermarket's while stocking two things for a very small market, even if that means they stock the less nice thing that serves a larger group of people.

Perhaps; but even if supermarkets didn't feel like stocking the stuff, I'd expect there to be specialist shops, or mail order, or some means of getting hold of it, unless the manufacturers were deliberately refusing to sell it on a non-prescription basis.