"MVPs do have to buy Y, S, B or Z fares in order to automatically secure a Premium Class seat at the time of booking. If you don't buy one of those fares, you'll be added to the Premium Class waitlist and cleared two days prior to departure, based on availability.
Golds are eligible for Premium Class seating at booking on all fares except for G, T, R, and W. For G, T, R, and W
fares, you'll be eligible for upgrade within 72 hours of departure, based on availability.
There are no fare class restrictions for 75Ks."

I'm guessing you purchased a G/T/R/W fare and now they want to charge you accordingly. But because you were involuntarily moved (which they have every right to do), perhaps you can request the fee be waived. In the end, you are never guaranteed a specific seat, just one in the class of service purchased.