Why do stores keep Betta fish in small containers?

Betta can't be kept with other betta, since they are aggressive and territorial toward each other. It is also highly likely there will be trouble when house them with other fish. Not necessarily that the betta will be the bully, but rather many nippy fish might nip at slow moving betta's fins. So it is the best to keep each betta alone. It is not economical for fish stores to have one full set up (fish tank, filter, heater, etc.) for each one of the betta. So they use small plastic containers instead. Of course, many of the betta die before they can be sold, because small containers with no filter and heater is no place for any fish. There is no way to get the fish stores to change this policy unless there is a law in place to prevent them from doing so. Money always comes first for a business.