Open source proponents get a bad rap when it comes to copyright. Often, they are called anti-copyright or copyleft. On some level, it makes sense. Open source software is advertised as being free as in freedom, and free as in free beer. Yet, when thinking about copyright law and restrictions, the last word that comes to mind is free. However, open source proponents are not actually anti-copyright. Think about it… They can't be. Copyleft or open source could not exist without copyright. Open source is actually a world away from what would truly be free in the world of IP, and that's the public domain.
Copyright laws enable the creators of open source software to make code available through an open source license (usually the General Public License). This license is what forces the users of the open source code to keep it open. In contrast, one could not force a user to make code in the public domain open even if one used that code to create a derivative work.
Thus, enforcing an open source license is really using copyright laws in full force. One is barring people from using code the way they might want to unless they agree to make the code open. That is clearly using copyright law to restrict the way one's work is used. It may bar some people from using the code at all. One might even argue that forcing code to remain open is more restrictive than keeping code closed, because people with closed code typically need not invoke copyright law. No one will ever have access to the code to be copied, so copyright becomes a moot point with closed code.
Thus, in reality, open source actually embraces copyright in its current form. It uses the protections of copyright more than closed source code does. It's not really anti-copyright. The only difference between open and closed source code with respect to copyright are the conditions being forced on the user. However, the restrictions are ever present in both. Open source restrictions utilize copyright to its fullest, giving the creator exclusive control over their work to keep it their code open.