As my other post in this thread gets into, this example of a "nudge" doesn't compare to the junk food case, since junk food is effectively designed to hack our brains' "food reward" mechanisms. Bag use is effectively neurologically "neutral", but junk food (and most other highly processed food) is very much not. "Pringles, can't eat just one!" turns out to be a lot more than a marketing slogan.