In my experience this is untrue. This past summer I helped my friend set up a small fish tank, and we went to Petco to get some basic supplies for the tank. When he was ready to get a betta for his tank over a month later, we saw a few bettas there that had been there during our first visit.
I suppose it depends on the locale. Just like there are bad mom/pop shops there are good big box stores. We also have a rather popular demand for them in houston.