I'm going out on a limb here, but I think if a promotion was sent to you for $10 discount tickets that were not next to your existing seats, you would whine and complain about how its not fair that you had to pay full price on your original tickets, and/or the additional tickets weren't by your existing tickets.

Furthermore, I seriously doubt that you would purchase the additional tickets even if you weren't highly offended at the promotion itself, because you already have tickets that are probably much better and have enough for the people you were originally planning on taking to the games. There is a slight chance that you buy the additional tickets for friends that have shown a desire to go to the games, but don't have the time/resources to buy season tickets. Of course, these people have never decided to buy season tickets before and going to one game doesn't greatly increase their likelihood of buying season tickets in the future.

However, by offering the promotion to past season ticket holders, these are people who have dropped hundreds/thousands of dollars on season tickets in the past. They have shown that they have had the time/resources/desire to be season ticket holders in the past. There is a MUCH greater chance of enticing these people to become season ticket holders.

Furthermore, I don't think the goal of the Athletic Department is to sell out every game, but I'll save that for another post.