Earlier quoted context omitted.
Gleaning was more about allowing the poor to have inefficient access to the raw materials that with hard work could be made into food. Taking prepared food from a merchant would have still been a crime. For example "People do not despise a thief if he steals to satisfy his appetite when he is hungry, but if he is caught, he will pay esevenfold; he will give all the goods of his house."
I'm not sure that the ancient Israelites had many places you could go get "prepared food," but the Bible also says you should leave behind grapes for the poor which clearly do not need preparation. e: Previously I claimed that olives don't need preparation which is false. Sorry.
Olives cannot be eaten raw.