GPL is &quot;clearly free software&quot; too, and FSF&#x2F;OSI approved too. The problem is that the GPL and the CDDL <i>both</i> have no-further-restrictions clauses, as mentioned in the article:<p>&gt; 6. …You may not impose any further restrictions on the recipients&#x27; exercise of the rights granted herein.<p>&gt; 3.4 … You may not offer or impose any terms on any Covered Software in Source Code form that alters or restricts the applicable version of this License<p>The situation would be exactly the same if, say, Linux were under the CDDL and ZFS were under the 4-clause BSD license, or the OpenSSL license, or Creative Commons, or the MPL 1.0, or whatever. The GPL and the CDDL both have exactly the same &quot;problem&quot; here.