Truth is if you free range your birds you could get by with corn and free choice oyster shells.
Don't be ridiculous (winter in Alaska, Montana, various other northern places.)
Of course you are perfectly correct (part of Hawaii where feral chickens are clearly doing fine with no human input, showing that they don't even need the corn and oyster shell.)
The problem with "free range" is that it depends on where the chickens are ranging, and what food they can find to eat. Climate matters a lot, and so does the amount of space, and how many chickens are in that space.