It is true that low quality feeds of both chicken feed, cat feed and dog feed can contain questionable ingredients.
Most pounds send the euthanized carcasses of dogs and cats to the rendering plant. Usually they are rendered down into a fat and then processed into "animal tallow" or "yellow grease" or "animal fat" or any other moniker it has as a proteen source for the feeds. This is absolutely horrible for dogs and cats (mad cow disease came from feeding dead cows to living cows-- cannibalism is a BAD THING), and while it might not sound horrible at first for chickens, you have to understand something about the way the dogs and cats are euthanized.
They are injected with a drug called sodium pentobarbitol... this drug is NOT destroyed in rendering plants heat, and so is effectively passed down, in however low concentrations, through the rendered products and eventually the products they are added to.
Purina has a soy based layer hen feed, and I use that. I only feed my dogs high quality dog foods where the "meat" sources are named and not quetionable (I also don't feed them grains either, since wolves don't plow, harvest or cook grains in the wild, but that's another story).
It's up to you to decide what you want to feed, but for me, I strongly prefer that my chickens not be eating sodium pentobarbitol tainted food.
ETA: I am not with AR groups by any means, and am actually vehemantly opposed to them... but this is something I have done extensive research on and I have found nothing to disprove it.