It is unfortunate whenever you are unable to participate in a race that you have registered and paid for. However, having been on the organizational end of a few large races, I can definitely say that it is just not economically feasible to offer refunds or deferrals, in most cases. In addition to the extra staff power it would take to process deferrals (especially in a huge race like at Disney), quite simply, your money has been spent. The food has been ordered, the staff has been scheduled, the bibs have been printed, and your medal and shirt are waiting for you.
Think of it as a wedding; if you call the day before the wedding to say that you can no longer make it, the bride and groom will still be paying for your seat in the hall, all of your food, your favor, and your slice of cake. This close to an event, there is simply no way to get back money that has already been spent. It would be nice if Disney would still mail your shirt and things to you if you can't participate, but I can see why that is more effort than it's worth. The charity I work with allows non-participants to come pick up their shirt from their office after the event, but will not give refunds if you can't participate. They just couldn't continue operating large races if they did. I know Disney is a big company that operates on a different scale, but the principles are the same.