As others said, KI is going to try to maximize profits, so they've surely taken the idea that sharing will happen (against the rules of the wristband, if there are any, or not) into account.
They're obviously targeting a certain demographic with the wristband. Most likely, me. I carry around the same bottle of water all day and fill it up at fountains. I don't need soft drinks, I have water. I actually drink a lot of water and there's no chance I would pay $30+ for the equivalent volume of soft drinks during a day. I am also not interested in carrying around the oversized souvenir cup.
However, I will pay $5 for a wristband that lets me get the 20oz drinks on demand. So, the park has $5 more than it used to have, at a cost of very little--fountain drinks are extremely cheap for them to provide. The only real question is how much they are losing from the people who would have spent $30+ on drinks, and are now spending only $5. I'm sure they've crunched the numbers and probably continue to do so.