To spread the word about proper betta tank size you can go on Petco and Petsmart's websites and write negative reviews of their bowls that are too small. Nothing rude, just something like "I found this bowl is too small to keep a betta humanely, and I had to change the water every couple of days. Bettas need room to swim and plants to hide in. Like other tropical fish they need filtered water heated to at least 78 degrees. I now keep my betta in a (name one of their tanks that's big enough) and he's a lot happier there!"
I spent years keeping bettas in half-gallon bowls too because I didn't know better and didn't have internet to look up how to keep them properly. I stopped keeping them for a few years, then recently when I got back into it, I bought another small cube - then I checked the internet. Now I have tanks for my bettas and the cube is used for one tiny male I bought as a female. People don't always want to be cruel or thoughtless, sometimes they just don't know any better, or they want a pet fish and can't afford the whole tank setup.