The problem with the exemption is that it requires staff to do the immigration and customs work. I'm not sure whether it would be the airport or the US government repsonsible, but whoever it is would end up deciding that it wasn't worthwhile putting it in place when there are no flights to the US.
Of course, the vicious circle nature of things means that not having it means airlines less likely to fly transatlantic, which means less possibility of getting it.