I try to do what I can and encourage people to speak up about mistreatment of bettas in stores. I strongly support education as the #1 tool toward the better treatment of all animals.
On topic: I have heard of Walmart killing the betta fish that are too sickly to be sold. I saw a terribly treated fish in a store and took a picture, sent Walmart a post on their Facebook. At first they left it up and directed me to the individual store's report form but then they removed the photo and post all together. They also did not correct me when I said that the fish was more than likely taken to the back and killed because it looked unsellable, since it was gone the next day. Walmart also makes money off dead fish by filling a claims form for a broken product, essentially. There is SO much wrong with Walmart and live animal care.