A couple of different vendors told me the reason is that the price on the ticket must correspond to the published fare (IATA?) otherwise the vendor will get in hot water from some governing body (any help from the travel agents would be appreciated). The two vendors was a TA in BKK and the other the airline's ticket counter agent in California who sold me a ticket. I used a discount cert in the latter case for an int'l trip, yet the full price before the discount showed on the ticket. She mentioned that the undiscounted fare has to show on international tickets but not so for domestic USA trips. That's the best loose info I've got.