To be fair, they are perfectly within their rights to sell products at their own prices and rules. I have to admit, if I owned it, I'd try to figure our how to keep the outside food out. Of course, if I owned a Taco Bell, I wouldn't want people bringing in a box of Pizza Hut, either. I mean, we call SDC a park, but it's not like it's a municipal park; it is privately owned. And incidentally, we carried food into Disney World just about every day we were there.
At the same time, if there's no written rule about pouring out your refill and getting another one, I don't think they have a leg to stand on. It's kind of complicated.