According to Professor Holaar, Digital Law Online: History.ยง I.E.1: "There is nothing in the legislative history of the bill that indicates why this change was made. Perhaps the best explanation is that Congress was concerned that people would make copies of software they had rented or borrowed from a public library, where they were rightful possessors during the time they had the software, and then not delete the copies when they return the software to the rental place or library."
