All property rights are "unnatural" (animals don't recognize property rights, only possession). In my view, copyrights are the least unnatural. We grant property rights in land with few restrictions, yet real property rights are a greater restriction on freedom than copyrights. A copyrightable work, unlike a piece of land, is the fruit of someone's labor. Land, in contrast, pre-exists the owner. And whereas land is a scarce natural resources, the universe of copyrightable works is unlimited. There is no good reason to attach more strings to copyright than to real property.