not all airports charge the max PFC (though most of the larger ones do). Further, they have to say what they will do with them. MEM currently has no PFC, and CVG lowered their PFC from $4.50 to $3.00 a couple years ago.
The lack of PFCat MEM has always been one of the reasons NW and now DL has pushed it as a connecting hub, since it is easier to compete price wise.