You're quite correct, the Open Core model (and, to some extent, the proprietary relicensing system as well) is made possible because of the way copyleft works. I've previously written that <a href="http://www.ebb.org/bkuhn/blog/2009/10/16/open-core-shareware... is only a tool, and the tool can be used for good things and bad things too</a>. I also agree that permissive licensing is <em>much better</em> than GPL+OpenCore.