I feel the same way, but it's a true ethical dilemma. I'm an environmentalist, and I think that buying duplicate cups is unnecessarily wasteful. But since there's no debate allowed in this thread, I just want to make it clear that I abide by the policy even though it makes me uncomfortable.
If one wanted to be both environmentally conscious AND ethical, could one pay for refill privileges and use an old mug? Disney would still get the same amount of money and you would not end up with excess mugs. Is that feasible?

But nonetheless that's creative and good thinking to save on snacks. I do the other thing with capped drinks though, will use my mugs, and grab a drink to pop in the fridge. Usually grab a chocolate milk or Orange Juice to have when I first wake up, take my meds, and ready to get moving.