I think the reason is that Disney is "lending" the $, but in essence they are not really "lending" $. They are selling you an interest in something that they already own, therefore there is actually no $ changing hands. This is not to say that there is not a cost to Disney. There is an opportunity cost in the sense that they cannot sell your points to someone else while you are using them, so if you were to default Disney will have lost the ability to sell your points earlier, but in reality Disney has not had to put up cash. Also, the risk to Disney is very low, because if you default the your points just go into available inventory that Disney can then sell to someone else, and they do keep whatever you've paid up to that point.