Chickens do not have the enzymes necessary to digest milk protein. Chickens can digest fermented(yogurt and keifer and etc.) but not very well. Too much will give them diarrhea and watery poops.I've heard this before but never had opportunity to ask about it.
Why do you say they cannot digest it?
That is why you give them just a little bit--to help with their digestion. Too much messes them up though. They should get 70% grains and the rest mostly greens and protein sources.
Most poultry feed is is best for them. Fermented feed seems to help them absorb the nutrients.