Maybe I'm misunderstanding: Are we talking about TS? or QS?
If QS, I'd be surprised to learn that single-serving containers of anything were ever "refillable". I'd have to dig back into the archives, but the way I remember the
DDP, back to the beginning, was that "refilling" of milk or juice as part of redemption of QS or snack credits was something that was never really intended to be - perhaps Disney simply allowed it for a while, despite it not being supported by the way the plan was offered, and folks over-exploited it, prompting Disney to crack down.
If you're talking TS, that's an interesting change. I have to wonder what would prompt this. It is small potatoes (as compared to no longer including appetizers, for example) and such that I would doubt they'd make such a change unless there was some abuse-vector involved. Were TS restaurants serving
containers of milk/juice? Or, if they were serving cups, were some guests filling their own containers off the largess? Either of those situations would make this change make a lot of sense.