NW decided that customers should pay $5 for the priviledge of booking over the phone and $10 for the honor of buying at a ticket counter. They also decided that GDSes should share the cost of booking a ticket; according to NW they always absorbed the approximately $12.50 cost of a GDS booking and decided to pass $7.50 of it on to the GDS. The GDSes responded by either pulling NW's fares, adding $7.50 to NW fares or listing NW fares last. Without the rest of the legacies matching the extra GDS charge, and losing lots of revenue (I heard a 17% dip in bookings through one distribution channel), NW pulled the GDS charge but kept the charge for the priviledge of using a human. AA matched. And now apparantly US has as well.

These guys can't raise fares, and in that regards getting $5 for ticketing via phone and $10 for at the airport is even better, since I doubt these fees will be refundable, nor will they be subject to the 7.5% excise tax (a $5 fare increase only results in $4.65 for the airline, as the rest goes to the govt.).
However, I can't understand why they don't do what AS, B6 and DH do, which is offer $5-10 off for fares booked on their websites instead of over the phone.