Really now. Wow. Hackers hack things because they can. That's all there is to it. Anytime someone creates a "hack proof" system, people will hack it. Just to do it. Remember the video game Doom? Well, there is a running joke to see if hackers can get a device to run Doom. If you can get it to do that, you can get it to do almost anything. Those giant display phones you see in some electronic stores? People have rooted those just to install stupid crud and then post the video on YouTube to show they did it.
RFID devices can be cloned in a fraction of a second. As quickly as the device can be read, one can be cloned. It's not that noticable. A proficient electronics person could hide the tech in something perfectly normal, be it a book, eletronics game device, something like that. All they need to do it walk by you and they have your info. Stick it in their bag, you would never notice. Check out this post from 2009 about what is involved - >
http://cyberinsecure.com/cheap-rfid-sniffing-and-cloning-device-for-sale-by-researcher/ For $250 they can scan anything within a 30 foot radius.
Now it is possible to do some things to lock down the data a bit, but if a company can create a certain device, another can create a copy of it.
Why would someone go through the hassle of doing this? Why would people make fake pins that then retail for $.50? Or buy non transferable tickets to resell? Profit. If someone gets a valid band, they can easily buy say 5 different DVDs to resell at a flea market or on eBay. Doesn't necessarily have to be a huge item, if anything smaller items are quicker to resell.
As a PP said, typically people try stolen CC's on a very small item, just to see if its valid. Try to buy a soda see if it works. If it comes back as no charging, "Ooops, grabbed my daughters bracelet." Pay cash and know that one is garbage, or say "I have to go get money, let me go grab some from my wife" and just not come back. It may even be easier, if they add NFC style payment methods on vending machines, no humans to interact with.
Is any of this really going to happen? Beyond someone hacking the braclets, probably not. I can almost guarantee that once the AAs read your bracelet to say "Welcome Johnny!" that someone will hack them just to record Peter Pan saying "Welcome to the 5th gate of h-e-double hockey stick!" and upload it to YouTube. As much as guests forget it, WDW is part of the real world, and bad things do occasionally happen despite WDWs best efforts to prevent them.