I was able to keep a goldfish from the fair for a number of years.
Plus, to play devil's advocate, people would be up in arms if you were buying kittens and puppies for animal food, but not if they were going to marginal homes. So...yeah. Imo feeding fish to fish seems more barbaric to me than exchanging the same fish for a pet. A large number of the wild fish collected for our hobby die during transport, but we don't care about those right?
Like I said, I'm playing devil's advocate. I feed fish to fish all the time. I also have an idea of how long feeder fish are expected to live without chemical treatment.