It's a fairly common practice and for shops its easier to keep the bettas in these than in normal sales tanks where it's hard to fit them in with the other species. It's not particularly nice to see but most shops that have these tend to have a high turnover of bettas so it's normally temporary. A shop near me used to keep them in little vending machine drink cups as did my local wholesaler, simply sat on top of a tank to keep them warm. Out where they are bred it's common to keep them in tiny bottles or jars (there are some videos on YouTube about this).
When I imported a few with other fish I ordered they can in tiny triangle bags that were designed to fit in the spaces between the other bags. They were in a tiny amount, thimble full really, and were always fine (although I only imported a few before I decided I didn't want to anymore).
So basically this is nothing new, it's been going on for decades and will always go on behind the scenes. It's quite interesting to look into the whole betta breeding world and discover exactly why the average 60 l tank is a luxurious mansion to them.