If you make it clear to them that you are willing to pay the price for the past Minny game, that's the only fair thing, as I see it. Otherwise, people would try to buy season tickets after the stupid preseason games all the time. Maybe that's the reason for the rule. Just to be cut and dried about it.
As a 28 year sth, who never uses my preseason games, I'd be a little pissed if people could buy seasons after the preseason was over.