Chickens love apples...nothing wrong with them eating apples.
Apple seeds, cherry pits and peach pits all contain trace amounts of Cyanide. So there are some people that will tell you not to give these items to chickens....but even almonds contain trace amounts of cyanide; I love almonds.
The trace amounts are so low, that if a chicken ate a 5 gallon bucket of Apple seeds, in one sitting....yeah it would probably reach a leathal limit of cyanide.
My chickens eat a lot of apples that fall off our trees. I'm not really worried about poisoning.