From the Joomla announcement :
Can I release an extension under a non-GPL license?
It is our opinion that most extensions are derivative works of Joomla! and must be licensed under the GNU GPL. It is possible that an extension could work within Joomla! and not be considered a derivative work according to copyright law but this would have to be evaluated on a case-by-case basis. If you believe your extension is not a derivative work we strongly recommend that you seek professional legal advice.
My emphasis in there. Basicly, the Joomla core team has decided that their interpretation of the GPL means they can require all developers writing code to their API to use the GPL. Not only that, but they can make that call for any existing body of work developed for either the 1.0 or 1.5 versions of Joomla.… Read the rest