I don't understand. The guy complain that json license is not "Free" in the FSF sense of "Free". So what? Not all licenses has to be FSF or FSF compatible. How is it harmful to FSF is the license of some piece of code is not compatible with GPL? Guess what, I can't use GPL in my projects, because it's not compatible with the kind of license I release my programs. You can't use this software because it's not comapatib…
Lots of GPL in-compatible licences are "Free" according to the FSF. Here's a whole list of "GPL-Incompatible Free Software Licenses": http://www.gnu.org/licenses/license-list.html#GPLIncompatibl... Some notable ones: Apache Licence, BSD licence, etc.