Copyright is NOT a natural right

"Congress shall have the power ... To promote the Progress of Science and useful Arts, by securing for limited Times to Authors and Inventors the exclusive Right to their respective Writings and Discoveries;"
(Article I, Section 8, Clause 8)

"Congress shall have the power" - as in they CAN, but they are not REQUIRED to. Modulo international treaties, Congress could revoke Copyright and be within the Constitution. If they had never created Copyright, they would have been within the Constitution.

From Thomas Jefferson, 1st Secretary of State and 1st head of the Patent Office (letter in 1813):

"If nature has made any one thing less susceptible than all others of exclusive property, it is the action of the thinking power called an idea, which an individual may exclusively possess as long as he keeps it to himself; but the moment it is divulged, it forces itself into the possession of every one, and the receiver cannot dispossess himself of it. Its peculiar character, too, is that no one possesses the less, because every other possesses the whole of it. He who receives an idea from me, receives instruction himself without lessening mine; as he who lights his taper at mine, receives light without darkening me."

As others have pointed out, until 1662 (1710 for the Statute of Anne that gave Copyright to Authors), Copyright DID NOT exist. So for 90% of Human History, you were free to copy to your heart's content.