It is to prevent a disease called blackhead that can kill a turkey but chooks are more resistant against. They can live on the same farm but ideally they should be kept seperate, the turkeys on land that chooks have not been on as blackhead is carried by a parasite that lives in the soil. Worming with Flubenvet works against the parasite but if you are serious about turkeys keep them away from chooks. Lots of folk keep them together with no problems but the risk is always there just waiting to happen. What makes the disease so horrible is it is quick and there is no cure so is not a good thing to be complacent about.