On the matter of eating poop: A common misconception is that B12 is produced by animals which is why we find it in meat, but this isn't technically true. B12 is produced only by bacteria. Many animals' intestines contain bacteria species that synthesize B12, but that B12 is inaccesible as it is not produced in the parts of the GI where absorption occurs most. In the case of herbivores that are not ruminants, the ingestion of cecotropes - special feces rich in these bacteria - is essential to obtaining adequate B12. That being said, chickens are omnivores and do not produce cecotropes tmk - but injesting fecal bacteria can still provide a source of B12. So technically, there is nutritional merit to feces in feed as the above poster mentioned.
That being said, I suspect it is far more profit motivated than health motivated (is there a cheaper nutrient source out there than poop?), and I'm not sure that feeding feces from countless other birds kept in industrial farming conditions is necessarily a wise practice. Especially in the age of increasingly virulent food borne pathogens, most of which are feces-based. Perhaps feces used in feed are subject to some kind of sanitization, but I would think if they killed the bacteria, they'd also reduce the B12 benefits - and we are increasingly seeing nasty, resistent, spore forming bacteria that defy traditional decontamination anyways. Either way, I'm glad poop isn't on the list of most non-industrial feed - I'm sure they eat plenty of their own poo pecking around for food, and if they're munching anyone's turds I'd prefer it be their own.