I agree with what you are doing, and I am a breeder and do cull out weak birds and allow "nature to take it's course" with any cold weather losses. You should do what your heart is telling you to do, remembering they have a heavy "coat" that you do not, so 70 degrees in winter for chickens is not needed, but if you feel better giving them heat, go ahead. I have seen heated roosts for sale recently. Those are intriguing, you might want to check that out as way to localize some safe heat for them. Give them an unheated roost also, so they can choose. I know which one I'd choose, but I'm not a chicken.
Chickens are descended from tropical birds that never encounter the cold we have here. They have adapted remarkably, but still aren't as cold-tolerant as our native birds, so they need some protection from drafts and extremes.
OTOH, ducks, geese and turkeys are extremely well adapted to the cold. Watch how they sleep, they tuck their head under their wings, have you ever seen a chicken sleep that way? Heat loss is highest from the head and feet, that is what the very cold tolerant birds protect when sleeping.
