Not everyone has access to chicken food w/ animal protein, I am guessing that is why they are doing it.
My chicken feed doesn't have animal protein either, so I'm not real sure how important that is, exactly. I choose not to get chicken feed with animal proteins because I'd prefer to not have recycled feathers and other CAFO offal in my food supply. Imagine the diseases in all that animal protein from CAFO sourcing? Yeah, I know they heat it and process it, but how much heat, exactly? Enough to kill the diseases from CAFO chickens?
It's fine to feed those mystery meats to the dog and cat because they are not eating the sickness of their own specie, but to feed it to my chickens when it comes from CAFO chickens/cattle/pigs I'm a little more reluctant to do. I'm not eating something that came out of my dog or cat, so I don't mind what they eat.