They actually did have a concept of land ownership though, didn't they? After all, the tribes had territories even before the Europeans came, they went to war with each other and fought over access to territory. Even within some tribes individuals or families had rights to certain areas - a family might own a tree and the produce from that tree was theirs, or sections of land for a garden or part of a river was the property of a certain person or family. Not all tribes had the same setup, and it wasn't exactly the same as Europeans thought of as owning a piece of land, but they did understand having the rights to certain territory.