The flaw with that thought (which the recording industry also used during the Napster trials) is that you are assuming that people would in fact make the purchase were they unable to get the product for free. With Napster it was circumstantial, at best (on an individual basis, though I have no doubt revenue was lost, it was just unquantifiable). However at Disney, I have no doubt that people would have more than like purchased a beverage, thereby decreasing Disney's revenue from soda sales. However, like the recording industry, Disney has no way of actually quantifying how much they have lost in revenue due to the abuse. There is no way of knowing whether or not people who had brought back their refillable mugs from a previous visit would have purchased new mugs.
Personally, I have reused my mugs, when I remember to bring them. At most I used them for maybe three or four drinks...barely breaking even with the cost of the mug. So this won't affect me (especially since I go to Disneyland more frequently since it ha the better parks), but I really foresee this as being a large headache for CMs by the number of complaints they'll get from returning guests. This is nothing more than a money grab by Disney. And while I have seen people re-use older mugs, I have only ever seen a mere handful of people doing it on any one of my 15+ visits. So while I don't doubt that abuse is widespread, in my personal (and limited observations) I doubt it really has much impact on the bottom line.