Because 15$ off tickets won't hurt them, and it's not that huge a discount. People like it because it saves them the price of lunch really. But as I've said before, in the theme park biz, we really don't make money from ticket sales - we make money from food and merchandising. The food prices and item prices will remain the same, so the more people we can get in the parks with discounted tickets, the more money they can make from food/things. That's why they offer the buy a day get a year around summer when its busy, and why you see the 15$ off admission around Grinchmas commonly as well.