It really is ignorance! Even experienced fish keepers sometimes skip over learning about betta care--I know for a fact from my boyfriend's dad! He kept bettas to start, but never learned proper care, aquarium hobbyists around him considered bettas an 'intro' fish, something you keep once and when you CAN keep larger tanks, you never look back. Who keeps bettas if you can house a pacu or an arowana? So though he researched and could tell you specific fish care of almost all other fish, bettas are still totally unknown, and he will proudly announce that they only need a tiny cup to live!
I am in the process of teaching him otherwise! He is starting to come around, seeing how active little Red Beard the pirate betta is XD he had never seen one make a bubble nest, so he was impressed at Nathan's 'skill' with these fishes!
It's the same with turtles! They used to sell Red Ear Sliders in these tiny little 'turtle ponds'... And now we know a hatchling needs a 30-50 gallon to start with a POND FILTER due to the amount of waste! :O Shock! With time, people will learn!
Although it would help if they would sell 5 gallon tanks a bit cheaper..I still can't get over the fact that I spent the same amount of money on my 55 gallon as this 5 gallon I want!