If there is no rule for OP UG for the following reasons:

1. prevent speculative bookings
2. reward the highyield coach pax or nice people
3. empower the airport agents (virtually a benefit)
4. upkeep the product integrity (either high price, or no price, just like making money off lottery or hedge funds: to increase the chance, you have to cough up big money to increase the odds)

NWA gives out their frontcabin more liberally, and pax have so many ways to get UPed, the end result is both pax and the airline are worse off ...