I've heard the same thing about lettuce. Its because of the high water content and no nutrional value. Mine have lettuce once in awhile. Its not like its the only thing they have to eat. Then it wouldn't be good for them. Like anything outside of moderation, can be bad. Apple seeds are not good for chickens. A few won't hurt them but if they were to eat an excessive amount, then it could hurt them. Raw potato peelings are suppose to not be good for chickens either. I do let mine have them from our baked potatoes though. I know there are more foods that are not good for them but I can't think of what they are right off. I'm sure Apyl and some of the others will bump on here soon with more info to add. Good question to ask allowing for a great learning opportunity for many of us. I am looking forward to comments others will have to include.
If it were in the garden they would eat it. I plant a row of kale for mine because it will live through most of the winter down here and gives them greens for the winter. Also they will eat some turnip greens. First thing mine do in the morning when I turn them out is start grabbing clover, grass and grit. Last thing in the evening they make a tour through the garden looking for low-hanging tomatoes and blackberries.
Everything in moderation. Raw / green potato peels are posionous, potatos are in the night shade family. Raw onion is not good for them, but some people still give raw onions. But just like the apple seeds its all about moderation. As for lettuce, its the iceburg thats frowned upon but its not like your going to be only feeding lettuce heads as feed. Its fine as a treat or in scraps.