Now you ask "why are Betta's often sold in small amounts of water", I've yet to discover this answer fully but I do find it slightly inhumane myself. It's no wonder why Betta's AKA Japanese Fighting Fish are aggressive when together after reading the above article and why they seem so sad when living alone. The same question should be asked to pet stores that keep too many fish stocked in a single tank at one time. You could debate that pet stores do this because there selling rate and providing a big selection for buyers, or you could debate that the fish you're getting from these fish stores aren't as healthy as they may appear. I believe if you want healthier fish that are kept in these conditions then go to a store where people actually come and purchase the fish and if you see dead fish in the tank with them at the time of purchase, chances are they may not be THAT healthy.
On the other hand I somewhat feel schooling fish or more non-active fish will do okay in a tank that may perhaps be a tad too tiny for them. Just insure your fish have plenty of room to swim and they're not living fin to fin.