They have "souvenir" mugs that you *can* refill, but you'll pay full price. And the cashiers will get really tweaky, telling you repeatedly that they don't have free refills on souvenir mugs (making SURE you know it).
(OK, at least ONE cashier, at WWS, got really tweaky with me, repeating it several times...maybe she's the only one, LOL...but really, you'll pay full price. I'm happy to be environmentally friendly and reduce the trash I use, so I was fine and assured her that I knew and didn't want a throwaway cup, but others won't want to deal with it)