IMO, it has nothing to do with ambiguity, and everything to do with certain people's need to want to screw with a system and put themselves above everyone else.
The sign seems perfectly clear to me. How can anyone argue NOT spending $14.90 on a trip that costs in the thousands? It's being cheap, and raising the prices for everyone else.
I am referring to the difference in wording between the mug and the sign and its implications on split stays, not at all about bringing old mugs back.
The wording on the mug implies you can ONLY refill that mug at the drink station where the mug was purchased, through the "end of your stay". The sign at the drink stations, however, don't say it is only for mugs purchased there, but for a mug purchased during the stay.
Now, with the question as to whether "end of your stay" in a split-stay is the end of the stay at that resort, or for the entire stay at WDW, still unanswered, take the following situation: I purchase a mug while I'm staying at POP for the weekend. Then I move to BCV for a week.
The mug's wording implies that I can still refill the mug while I stay at BCV, but I have to go back to POP to do it... the sign on the filling station doesn't limit me like that, as long as I purchased the mug during the current stay.
The split-stay issue has long been unanswered.
Frankly, if Disney wants to increase sales of mugs, they should make it more convenient to use them. To have one fill station in an entire resort, especially the more spread out ones, makes them less useful to those who who are far from them. On our last trip, we would have considered getting up to 6 of them...except we were at the extreme end of Grandstand area of SSR, and it was far too cold to be walking all the way to AP to fill them...