The important thing to understand about fish is that they can't just live in tiny cups. They need space, filtering, food, etc. It really wouldn't hurt to give Betta fish their own tank in pet stores. The problem is that they can be aggressive, however you don't know how long the fish will be there. Keeping a fish in a bag is fine- granted that it's only for a very short time and you'll put it in a tank afterwards, however Betta fish could be in the pet store for weeks for all we know, and obviously they cannot keep living in a tiny cup. An adequate living space is a very basic need for a fish, and a plastic cup most certainly does not meet that need.
Betta fish can't just be living in small cups. Many of them die in the cups and we need to give them a life they deserve. The cups are filled with dirty water and poop, and the ones stacked in the back will barely get adopted if you can't see them. Let's stand up for all betta fish and get them out of their prison!