I definitely understand your point, especially since I'm vegan and deal with this issue all the time. However, I don't see how refusing to buy a sick betta does anything for the fish. I would LOVE to change the world for animals, I really would. And I tell people all about my lifestyle, wear animal rights t-shirts, attend protests, leaflet, and do everything I can to help the cause. But I know that I personally can't change everything on my own, and that it will take years to make the difference us animal lovers want to make. So in the mean time I want to help by making individual lives better, even if it's just a few fish, I know that I will have made a difference.
Kindness and respect should be shown to all animals, no matter if they are furry, scaly, big, or small. Be a hero to the animals and GO VEGAN!