It wasn't a stunt.
It was still a marketing stunt albeit a well-meaning benevolent one-shot stunt. If it wasn't a stunt, they would've made it a permanent fixture on the menu.
They should been selling them all over the U.S. for Earth Day, if the plan to make and sell them had gone through.
How can you be so naive? You really don't understand how franchises and businesses work.
All over the US? That would mean the corporate headquarters would have to force franchise owners to participate. And they more often than not, don't force (or can even try to force) 100% participation. Many locations are independently owned by third parties and not by the brand's parent corporation. Ergo not every branded restaurant would want to participate because of the added expenses (in terms of technology needed, training needed, logistics needed) to pull this very limited marketing ploy off. Politically speaking, there would be some franchise owners who don't like the Peace Day collaboration either because they don't want to buddy up with the competing brand or they think Peace Day is some kind of Commie or Hippie conspiracy.