If you were to house the feeders in a seperate tank, feed them very well for a few months and then feed them to your fish, that would be different. Fact is that feeders are shipped to stores in overcrowded, ammonia filled bags. They are then often placed in tanks too small for their volume and not fed. By the time they hit your tank, they are starved, stressed and in many cases, days or less away from dying on their own.
In the wild, many fish are more opportunistic in their eating of live fish. They pick off babies, injured or diseased fish, while primarily consuming vegetable matter, fruits, insects and anything else they come across.