To prevent them from having to allow you into the park after there have been significant price increases. Giving back the money, is a partial thing, but, unless they include interest on that money they are still ripping everyone off. They make it sound good though... "gee, we are giving you your money back so it's not like you will lose anything". I have very secure investments and still manage an average or 5% return. So if I invest $400.00 (+-) I want 5% more per year then I paid otherwise they are stealing my money. Then, I'm sure that they will allow me to repurchase another ticket at the higher price.
There are also accounting concerns, but, that is for their convenience not ours. Outstanding tickets are considered liabilities in accounting and are recorded that way and they artificially considered debt. Something they have to pay off, via services, in the future.