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?