At some point of time, however, I believe the mug policy was that you were able to use the mug on any subsequent trip to the resort. For instance, our family bought 2 All Star Sports mugs in 2006. Now, during the time of purchase and today the rule has changed. But, to each its own.
However, I saw people during our stay at POFQ in December using mugs from the 90s from other resorts! I'm not trying to argue that this is right, but if Disney really had a problem with this, they would crackdown on the issue. Most likely, the people who use these mugs return to Disney regularly. For each of these families, there's probably two who just bought the new mugs. Is Disney losing money? Absoultely. Is it worth the trouble to enforce this rule? Probably not.