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. Ignoring the argument about how companies could be “scared off” by this (frankly, unless you’re talking about very small companies that can’t handle service contracts and maintenance, I don’t think that’s an issue – service contracts net more profit than retail does for this sort of user base), the thing that’s most annoying about this is that frankly, they don’t have that right.
Yes, Joomla is a good body of work. It’s not by any means the only open source CMS out there, and there’s a lot of people who’d argue over the assertion that it was the best. But if you put in place a restriction on how third-party software can be developed for it, you’re going to limit its growth and from the core team’s point of view, that’s shooting yourself in the foot.
Clearly, the Joomla Commercial Developer’s Alliance agrees. From their statement:
Whilst we appreciate the clarification made in this statement we deny that extensions that are designed to work with Joomla can be considered derivative simply by their nature. Our advice and research indicate that there is plenty of legal argument that supports this view
Never thought I’d think a group of for-profit companies would have a better grasp of the GPL than a group of open source developers!