So you think most guests spend wisely? For the last 20 years you must not have been involved in business or read a great deal about how business, advertising, and credit are affecting the average US consumer. Your average American doesn't save properly, are impulse buyers, and overspend on vacation.
Businesses place a cardboard stand with junk on it in the middle of an aisle people walk down and the item increases sales by a huge margin just because it is there. Look at your local grocery store checkout lines. Notice all the absolute junk you are surrounded by? That is because those products sell like hotcakes there because people and their children see them and buy them. Why because it is there and easy to get.
Amazon added and promotes "One click shopping". Why, because people WILL buy more if it is easier to do so! This is a pretty basic principle of business. Make it visible, make it easy. Sure, some people don't buy that junk, and some people resist easy. But not your average consumer. And especially not on vacation.
And there has been much written on vacation spending habits. Vacationers are notorious for loosening the purse strings, being less controlled, and doing more impulse buying. Yes, "many" people arrive at Disney with a budget and stick to it. But most do not. Most relax and worry about the tab later. This is well known. There have been many articles about trying to help people resist overspending while on vacation. Why? Most do it. There really isn't any question about this in business and marketing. It is why every vacation destination fills itself with souvenir stores that sell junk. Why? Because on vacation people will buy junk and lots of it that they normally wouldn't.
It boggles my mind that anyone is even doubting this absolute marketing truth that is well know. Just Google "Vacation overspending" and you will get 293,000 hits, the leading ones all about how to help you avoid the COMMON PROBLEM OF VACATION OVERSPENDING. And Disney is the expert at both making you feel fantastic and relaxed, making it easy, and giving you lots of opportunities to spend.
A little side point: It is easier (And kind of cool and fun) to use a band rather than the card. Also more people are attaching credit cards to the bands.