Seems like an odd, or certainly unexpected, change of direction, moving from designs that are used for year(s) to a design that is seasonal and is "good" for 2 months, 3 at the most.
This part of the Food Blog post jumps out at me:
"it is unclear at this time as to whether guests will be able to choose the current Refillable Resort Mug design instead, or if the Halloween version will be the only one available."
My total speculation: the same mugs we have now continue to be sold at the current regular price of $18.99 pus tax. But hey, you want to use a Halloween mug while you're here? You can get one of those for $20.99 plus tax (or whatever upcharge they think they can get).
If you're on the dining plan, you can use your dining plan credits for the Halloween mug, plus the whatever the extra amount is out-of-pocket.
Brilliant!