Wow, pretty interesting, I went to one of my better LFS today, and durn if they didn't have bettas that had just come in, packed the same way. I asked the guy how they actually survived, and he said he believed it was because for a temporary ship, enough water to cover and the rest oxygen seemed to be enough for them--he's not too happy about it either, as he is a breeder of fish as well and just works there, but has to follow store police. As Li pointed out, with such low price points (and still high profit margin) the bettas become "objects" to sell, not living creatures.
If they were in the gas permeable plastic I think there would be a host of other problems, the least of which might be the plastic sticking together.
The only criterion which perpetuates the sardine packing of bettas is their popularity as an easily replaced, expendable decoration for people who mostly don't know any better, don't think about it, or just don't care. For all their beauty I don't think I could ever bring another betta home unless it was privately bred.