From what I think I remember, it was the US Airways system and after the merger, they voted it in. It sucks for the guys at the top of seniority, but it does force people to list pretty early (not like United where people list last second as the battle it out with their vacation passes). Also, it benefits those who are trying to non-rev on multiple legs since the time of checkin is allowed 24hrs prior to their first departure. So imagine someone leaving Europe and connecting in PHL and going to the west coast. They will be checked in for the PHL-west coast leg 10 hours before anyone who is just originating in PHL.