While the signs have not been redesigned (why spend the money?) when the mugs were redesigned the words printed on the mugs were changed from the very clear refills "at the location purchased" or something like that (I don't have my mug in front of me) to the much more ambiguous "our resort beverage islands" (note the plural).
WDW wouldn't have made the wording more ambiguous if they cared where you refilled your mug.
The mugs wouldn't have been changed from resort-specific mugs to generic mugs if WDW cared where you refilled your mug.
CMs wouldn't tell you that you can refill the mugs at other resorts (as has been reported here) if WDW cared where you refilled your mug.
However, for some reason there are people on the DIS that can get whipped up into a fenzy when this topic comes up.
Personally, I think it is just another quiet perk that WDW provides its resort guests but isn't going to go out of its way to advertise. It's nice for the very few people who will actually carry their mugs with them when they are visiting another resort but costs WDW practically nothing.