Just in case no one has thought about it, there is a way to eliminate waste from the TS & CS huge portions.
(1) DH & I share a lot, even on
DDP.
(2) Get takeout boxes & take the excess food back to the room; we do this a lot. Store in the room fridge. When ready, use the microwave in the resort food area.
(3)If we have leftover DDP credits, we give them away or,
if driving, we stop by Publix, buy a $4 styrofoam cooler & take home our goodies (live in Indiana)! Once we even took home 2 full orders of ribs from ESPN that we bought on the last day with our leftover TS credits (once home we froze them & later cooked them with beans in a crock pot - ummmm, so good)! We used the resort ice to fill our cooler. We always have snack credits left, so on the last day we use them all on Magic Cookies at POR & cupcakes from the Starring Rolls Bakery in the Studios - yum, yum! Our leftover counter service credits are used at Studios Starring Rolls for sandwiches.
I HATE wasteful greed & manners also, but I don't always assume leftover food is such. There are other reasons as already mentioned. For instance I have often purchased apples at the grocery, only to take that first bite & discover they are "mushy"! Maybe that is what happened?
However, one thing I know for sure is that what the OP observed is not a Disney restaurant or Disney visitor thing. If one is interested in doing so, such can be observed most anywhere.