Another interesting food for thought regarding the ethics of "mug abusing", is to think of some of the principles of real estate law. For good or for bad, there is a thing in real estate called an "easement". Let's say that I own a piece of land and someone else owns the piece behind me. They can access their land by going around, but it's quicker and easier to cut across my land. I don't have to allow this, since it is my property, and I can put up "no tresspassing" signs, and it is up to me to enforce it. If, however, my neighbor cuts across my property to get to his, even with the signs, and I continually look the other way and don't enforce my "no trespassing" policy, eventually the law regards this as a tacit consent for him to cut across my property. And if I then decide after a certain amount of time (say 2 years, 4 years, or whatever the law or a court determines to be the right amount of time), to start enforcing my no tresspassing signs, the other guy can take me to court and claim that I have given him an "easement" insofar as I have allowed him to cut across my property for this amount of time, and that amounts to a defacto consent for him to do so, regardless of the "no trespassing" signs on the property.
If I own an empty piece of land and someone decides to build a structure on my land, I, of course, can shut him down since it's my land. However, if I tacitly allow him to build his structure on my land by my choice not to stop him from doing it, then I can't come back a year later and tell him that he can't have that there.
There are certain principles in the law which state that you have to enforce your ownership or else you forfeit it after a certain amount of time.
Similarly, I think we can look at Disney's mug policy and conclude that after this long, if they were really concerned about mug abusers, they would have done somthing about it long ago. And their failure to do so constitutes a tacit approval of people bringing their old mugs in.
Sure, it may not be a perfect analogy...few analogies are. But the point is that there are certain principles that are pretty well embedded in our society, that you must claim and enforce your rights over something, or else eventually, one can reasonably assume that you have forfeited your claim. If I am sharing a house with someone and I move out and leave all my belongings, I am only allowed a certain amount of time to claim my stuff until it is legally considered abandoned and fair game for anyone to sell, to keep, or whatever. Again, perhaps not QUITE the same, but I think the principle could still be considered to apply here. Disney has an official policy of not refilling last year's mug for free. It is up to them to enforce it, and if they consistently choose not to do so, then isn't it reasonable after a certain amount of time, to conclude that their choice not to enforce it essentially constitutes an approval of mug reuse?