It's about how copyright law was originally conceived. It did not particularly care about what you did with the information, apart from when you copied it. Copyright is only about the right to make more copies of a work (hence the name!), it doesn't give you any particular rights on its own to what anyone does with any given copy of a work, such as reselling it. This has changed more recently with (first with software) the concept of buying a license to a work as the primary means of purchasing it, which is far more restrictive.