Honestly, if I see a betta that's in need, and I have the resources to care for it, I'm probably going to take it home. Whether I buy it, throw a hissy in order to get it for free, shove the cup in my purse, or otherwise. Different people have different opinions on the matter.
As deplorable as some stores can be, others in the same chain may not be as bad. Petco is always going to order bettas. It really doesn't matter if you buy one or not. And unless you say something, it probably won't change the conditions they're in. All I care about is the well being of the one in need. You can't save 'em all, but I'd prefer to change the life of one, rather than do nothing. Even if I have to buy it, and support a store who maybe has issues caring for one species of fish, but has so many other great things that they do for other animals.