It's really sad. I remember before I had bettas, I would see cup after cup filled with dead bettas. It was heartbreaking.
I don't think they'd euthanize them, because euthanize could cost to much, euthanize fish after fish after fish. When i wanted to put down my sick hamster, the vets wanted 65 dollars to put him down. Many of the pet stores are pretty cheap (as in they don't want to spend money where they don't have to) and I doubt they'd pay to euthanize tens, if not hundreds of fish a year.
its sad. :(