There is nothing wrong with feeding feeder fish to true piscivores.
The feeders themselves then need to be well cared for and fed a healthy vitamin enriched diet as well. The big issues are
1. When people feed feeder fish as a major part of their diet to fish such as Oscars who do not really feed like that in the wild.
2. When people plop a 29 cent feeder into their prized show tank the second they get home from the store because somehow, magically, the fact that that fish is "food" means that you don't have to quarantine it like you would every other fish you would put into the tank.
And on a personal note I've had a lot better luck with healthy Rosie Red feeders than the disgusting goldfish feeders at most stores.