unfortuetly petstores have to keep the male bettas in small cups, because you cant generally place them into community tanks safely.
Also bettas were bred to live in small rice fields, it is safe to assume that they dont mind smaller invironments. *these cups are only tempory* keep that in mind too, usually bettas sell pretty quickly, more so when there is a two for one fish day.. I dont think housing male bettas in a cup is really a bad thing, as long as you clean them by-daily, and make sure people dont get the bright idea of mixing the cups together (it happens).
Because bettas come in shipments of around 20+ at a time, it is hard to have that amount of larger habitats available, especially if you are a small petstore retailer..