Bettas are amazingly hardy, adaptable fish. They can survive well outside of their ideal pH, live in tank conditions that would kill any other fish, endure a striking amount of abuse, and subsist on the poorest of diets. Unfortunately, this striking ability to survive rough conditions has resulted in stores and fishkeepers alike using the betta's hardiness against it for the sake of their own convenience - small tanks, no heater, infrequent water changes, no filtration, poor breeding; you name it. As a result, many bettas in pet stores or from a neglectful home are on their last legs by the time they reach someone willing to invest the time, money, and effort into properly caring for them. The result is sickly bettas.
I run a betta sanctuary, and I've taken on bettas dying in stores from columnaris, septicemia, emaciation, fin and body rot, parasitic infestations - conditions that would quickly kill any other fish. Almost unfailingly, I have healed this fish, and the overwhelming majority have lived healthy lifespans since then. I'd like to see someone even try that with most other tropicals.
So, yes - when properly cared for, the betta is an amazingly healthy fish. Often, even when improperly cared for, he is a reasonably healthy fish. Only when exposed to the horrific neglect he recieves in stores is he a sickly fish. The fish you see on this board largely come to caring individuals as "rescues" from pet stores, meaning they came from the latter - hence the large number of illnesses reported on this board.
Side note - I agree on avoiding pet stores when possible. Some pet stores have healthy fish, but I can almost never find a pet store with healthy, well-cared for bettas. Some breeders are reasonable about prices, and some will even let you pick the fish up at their residence if they live nearby to avoid shipping stress and cost. Well-bred animals who have been carefully and lovingly raised by an enthusiast will ALWAYS be healthier than poorly mass-bred animals who have had to fight for survival to make it to the pet stores.