At my Petco there's a sign under the bettas, which I don't remember vertbatim but something like "Did you know? Bettas thrive at home in water about the size of the containers they are sold in petstores because it is close to their natural environment, the tiny mud puddles in Thailand."
Not kidding. It really says this, and is endorsed by Petco. No wonder people have such false information about them.
Not to pick on Petco- And I will say that at this particular Petco the employees (while misinformed re: bettas) have been very receptive to my gently educating them now and then.