Just because rice paddies have low oxygen doesn't mean the bettas are living in their own waste like they would be in a small bowl.
Also, a small bowl is still going to have less oxygen than most rice paddies because rice paddies are exposed to the open air, have a breeze blowing over them, etc.. There's actually oxygen in the ground too.
I don't have any fish at the moment, but I don't see why anyone would want a fish in a bowl anyway, how boring is that? Boring for the fish AND the owner IMO.
http://www.bettatalk.com/ for anyone that's interested, I found the "betta care" section on this site very informative.