One thing that strikes me as unnatural about copyright is how difficult it would be to enforce effectively. With a traditional crime like robbery, the victim suffers a direct loss. The victim or a relative can report the event to the authorities. With copyright, the victim suffers no damage. Most of the time, the victim of infringement doesn't even know that the event occurred. To enforce copyright effectively, all private communications would have to be monitored at all times. After all, any data transfer between two people is a potential violation of the copyright of a third party. Even private chatter inside your home would have to be recorded and analysed. Otherwise, you'd be allowing kids to sing "Happy Birthday" without a license.