I don't know about you, but many times I've gone to a WDW store and bought something small, like a sealed pouch of cookies that I planned to eat right away. Without asking, the CM puts it in a shopping bag that I really don't need or want.
Or maybe I am in Disney Springs, and I buy something like a shirt. the CM puts it a more-then-ample shopping bag. Then I go to a second store, and buy something small, like a keychain, an item that would easily fit inside my first shopping bag. Often, even if I expressly SAY I don't want a second bag, the CM will STILL try to put my keychain in a second bag.
Now WDW is slightly different, because you are in the parks all day, but here at home, I often try to opt out of using a shopping bag. If I go to the pharmacy, and my prescription is already stapled into a paper bag, and I only have ONE prescription, why on earth do I want the clerk to ALSO give me a plastic bag.
If I can easily carry a few small items all around the whole store without a bag while I am shopping, then I can most certainly carry them another twenty feet to my car without a plastic bag. Yet sometimes it is very difficult to opt out of the plastic bag.
Maybe WDW's motives aren't pure, but many of us have gotten a little bag-crazy.
If we get BOTH store clerks and customers to think about every bag they use, that's the first step. Maybe the second step is something along the lines of the refillable mugs. Free refills if you wash your own cup! Maybe give WDW shoppers a small reward for bringing their own shopping bag.