When we buy these adult Bettas and give them good conditions it can be stressful on them. Changes in water temp/quality can be very stressful on fish just as bad or worse than dirty water. I think the hardier ones that survives the "bad" conditions is due to how consistently bad it is. I think more Bettas would survive overall and lead better lives if they was given good conditions from the beginning.
It is pretty easy to pick out the "hardy" Bettas as most of the more sensitive ones will die or look sickly pretty quickly in those cups and not be bought. So your experiences is that you get a Betta in bad conditions that can live in it and keep it there never gets stressed or sick. When someone comes home and gives it the "ideal"environment right off the bat can be stressful and cause the fish to become susceptible to disease and infection. I personally buy my Bettas from the local fish shop now as they get the best conditions and when I get them home they will not freak out on me.
continuing that theory, i believe a lot of people on this forum are breeders, who buy their fih from other breeders, who take better care of their fish, so its not such a shock. i too was at first skeptical of the ideal environment for the same reason listed previously. but then i though to myself... these are people who have owned a LOT of bettas, who have a LOT of experience... more so than pet store employees. and ever since then i check this sight every day to see what else i can learn about my little buddy :)