It's not necessarily the case that no-one has selected their seats (although it's quite possible). UA knows exactly when that conference is, as they deliver a huge amount of traffic to it. They're also well aware that many of the people traveling will pay C/J fares (especially with corporate discounts). The mere fact that only J or J and C are available doesn't necessarily have anything to do with the number of seats they've actually sold; it's an indication of the number of seats they think they'll sell at those prices going forward. And, I tend to trust them, particularly in this specific case.