Unfortunately Betta are a victim of its own success. Its ability to live in relatively small bodies of water even puddles has been misconstrued over the years and people with limited space get convinced by the LFS that they are fine in that environment. That couldn't be further from the truth, for sure a human being could be kept in a glass box if you fed them and found a way of removing the waste products, it would probably take a very long time to die but their death would be a luxury.
Betta have evolved with a labyrinth organ so they can survive in low oxygen water but not thrive. They do tend to live in small bodies of slow moving water unlike many other tropical fish we tend to keep preferring to keep out of open spaces with fear of predation, these small bodies tend to dry out right down to nothing and they have adapted to survive for longer in the hope of making it until the next down pour or if all else fails to try and make it out of there by jumping or crawling to some deeper water near by. It's not that they want to be there, they have no choice.
Obviously it's a fine line with store keepers who have to marry up the fact that they are good sellers and what you would deem as being ethical. Also compounded by the males can't be kept together. At least they should all have their own tank mixed with some calm species that don't nip fins. When space in the LFS is at a premium you can see why they play on the fishes ability to endure hard times as a way of achieving this. Putting them in yoghurt pots like sweets in a candy shop is the worst of both worlds for the fish, it puts it in a place it would much rather wouldn't be and takes away its security of places to hide and cover. People see these and think well if its ok in there its going to love living in my jar but really that's just fire to frying pan. You could put a dog on a tread mill and it will get exercise but it needs to explore, smell and have external stimulus to be truly living.
To the OP, I'm not suggesting you're being cruel to the fish BTW. What you've done there is provided clean water, cover and some plant life so the fish is probably quite content right now, its probably seen a lot worse in its life but I would suggest get a little tank for him when you can. Doesn't need to be anything fancy even just a little plastic one that holds a few gallons. A heater would be ideal to stabilise temperature and you might even get away without using a filter if you plant it out enough and change a little bit of water often. I think the fish will love it and more importantly so will you watching him living the dream.