Have they gone to bar codes property-wide then?
Unless you have a split stay, I'm not really sure how many people find a benefit in the mug being refillable at more than one resort during your stay. Maybe if you went to one of the MK resorts to rent boats you could benefit from bringing your mug and staying hydrated.
We'll probably get some use out of that change, since we tend to take two of our mugs with us in the morning (DH has to have his coffee and I need my tea) and then tuck them in the backpack when they're empty. I could see us getting refills at the BC as we walk from Epcot to MGM or at the Poly after dinner for while we watch Wishes from the beach, since we'll most likely have the mugs with us anyway.
Mug designs that change annually would help with enforcing the "length of stay" provision. YOMD this year, something else next year, etc.
My guess is that the year is now on them to prevent subsequent use in the future. We were able to use ours at other resorts. All cups were the same except for people that had the older ones