I'm the biggest plant fan, and when you have plants, magic happens. You can keep a fish alive and healthy in a quarter gallon jar. When you have plants, 1 gallon tanks become almost zero maintenance :)
So to me, the question isn't "what tank size can I keep a betta fish alive in" but rather, "what tank size is *ethical* for my fish?"
I believe the correlation between ethical fish keeping and tank size largely varies from fish to fish. Especially for the betta fish due to their unique personalities. To me, putting a nervous tail biter in a 10 gallon can be equally as cruel as homing a hyperactive plakat in a 1 gallon. A fish who needs a school should be given a school and the tank size suitable for their schooling activities, and a fish who demands territory should be given adequate space to reign over. In my opinion, the size of the tank should depend on the individual betta's personality.
That said, what would I tell a beginner fish keeper who just got home with a carnival won bettafish? I would tell them that five gallons and above is best and to go no lower than 1 gallon. They have their own fish keeping adventure to start out on, we can only point them in the right direction by sharing our own experiences. Those who care for these little lives will do the best to keep them alive and happy, While those who couldn't care less about the happiness of the fish, will opt for the cheapest way out.
At the end of the day, it's impossible for me to change the way someone thinks or the way they act. It makes even less sense to become upset about it. As long as I can keep my own fishes happy, I'm satisfied. Because to me, this hobby is meant to be a gift, not a headache :)