I think tanks this small are popular for a combination of two reasons.
The first reason is misinformation. The first person who sold me a betta swore up and down that they don't need a lot of room and can be kept in tiny bowls. I learned over time that the clerk was full of crap, but it took a lot of research and a few sick fish for me to realize it was my lack of care and improper housing that made all of them ill and inactive. Not only do few people bother researching fish before getting a pet but they assume the pet store staff knows what they are talking about. So, I know from my own experience that a shopper will believe the Petsmart or Petco tank suggestions blindly.
The second reason is the price of bettas. I think some people are under the impression that cheap fish don't live long and are supposed to be boring. If bettas were really beautiful fish with a 5+ year life span they, would sell for more than $5. So, they put the $3-5 fish in a $6 dollar tank because the fish isn't 'worth' an expensive tank. A fish that cheap is just going to die soon anyway, so why waste the money. *eyeroll*
I know my second reason is a bit harsh. But I've heard, "Its just a fish," too many times to not jump to that conclusion. If the store clerk is giving someone bad information about a $3-5 dollar fish, that person isn't going to do hours of research to make sure the staff member isn't lying through his/her teeth. If some pet owners looked past the bettas' $3-5 price tag, small tanks like this would never be used as permanent homes. : /