I think if they are worried about refunds, it's dangerous to allow people to purchase a month in advance. Are you saying they would allow for the booking of passes in advance also or just the booking of the service? I think operationally it would be tough for them to allow people to book the actual rides ahead of time unless they severely limited the number of these sold.
I don't really have armchair solution here... Perhaps they can do better by simply limiting purchases, then finding a way to add some additional value to it (snack credit, reserved fireworks spots as G+ options) and raising the price. At $15 people may add this every day, but if it were $30 I think you pick your spots. It just can't be the same exact thing for double the price.