My concern with your proposal is the suggestion that people would be doing the wrong thing by exercising their rights under a licence.
/* This post has turned into a bit of a diatribe, so I apologise in advance - I should have spent the time writing on the underlying legal question but I didn't expect to write so much on this! I think it is important, however, to consider the underlying issues for any discussion on an issue of this magnitude */
As you no doubt know, the model that a lot of open source developers follow is that if they build an open-source extension then it is likely that a sufficient percentage of users will pay the developers to implement it/extend it rather than do it themselves or pay someone else (less expert in the extension) to do it. The developers get their revenue from these installations and enhancements, ie services, rather than licensing a base application they have written from scratch (selling widgets). Because it is free, a level of viral marketing kicks in to aid the broad distribution and adoption of the software and therefore increase the market for support/development services.
Of course some clients will pay developers to build open-source software to fill a need that they have and which they are happy to share either out of the goodness of their hearts or in the hopes of establishing a standard platform.
One of the big reasons the GPL is written the way it is (as distinct from the BSD and other licences) is that philosophically the FSF wants people who build upon "donated" code to have to similarly "donate" their code, rather than create proprietary products that incorporate or are dependent on that donated code. The FSF does not believe software should have owners http://www.gnu.org/philosophy/why-free.html
- see the section on Economics in particular.
It is correct that hosted applications are certainly one way for developers to make money from modifying/extending GPL software as they don't have to re-distribute their modifications/extensions if they are run on their own servers (at least under the current GPL). Some people think that is against the spirit of the GPL too though, so to stop this happening the Affero GPL http://www.affero.org/oagpl.html
extends the GNU GPL by requiring providers of hosted applications licensed under the Affero GPL to provide a download link to the source (including mods) - see section 2(d). The great CiviCRM application/extension uses that licence.
The share-and-share-alike nature of the GPL is the foundation that has enabled open source communities like Joomla! to thrive (imagine if Mambo was not licensed under the GPL). The GPL therefore helps create the fabric of Joomla! society much like our Constitutions and Crimes Acts help create the fabric of our physical societies. Discussion of changing the licence therefore cannot be raised lightly.
At the end of the day, whether or not the society envisaged by Richard Stallman at the FSF is the one that Joomla!'s developers, contributors and community really want, is up to them/us! I certainly know that I would generally prefer to pay someone to develop an open-source extension than to license a proprietary one, and our clients appreciate the cost savings they get because the developers/funders of open-source software are so generous.
One way of supporting open source development (in addition to taking action against those who are breaching the GPL or choosing not to buy non-free stuff) is for the collective to support developers of open source projects financially through donations (The Dunc-Tank http://www.dunc-tank.org
was an attempt at this for Debian). If you google Dunc-Tank you will see that this is fraught with issues and really requires a far more transparent governance framework than Joomla! has right now, so that people don't think that there is only a special class of (core) developers who are in the club and get paid, while everyone else are second class citizens (regardless of their merits). I understand that at present the donations to support Joomla! go to the OSM and are used to cover developers costs attending conferences, running conferences etc - not paying them directly.
If developers of the open source project earn their income through licensing of non-free extensions, there is a risk of conflicts of interest as those core developers then have an interest in not enhancing the open source product in a way that would reduce their revenues. If the development team is open enough to enable new people to come in and build that functionality, then that is less of an issue for the community of users.
At the end of the day, no one can be expected to do something for nothing and therefore it comes down to how society (both broadly and within the Joomla! developers/community) believes the developers should be rewarded for their efforts. If it is through fame only, then eventually developers (who aren't already independently wealthy) will have to convert that fame into food and water by diverting attention from the project to paying work. That is the natural attrition you tend to see in many open source projects which gives young developers the opportunity to cut their teeth, step-up, prove themselves and move on.
Of course this is a bit of a rant sprouting from the big question underlying all this - Whether commercial non-GPL extensions to Joomla! are legal or not, to which the answer is - it depends :P - commercial extension developers to any software (including "free" software) should obtain their own legal advice
(and ianmac's post while I was writing this emphasises that point, so developers don't have false expectations) Whether or not non-free extensions should be encouraged by the community, even if they are legal, is a question that only the community can answer (through paying for them or supporting open source alternatives instead).
The above is a general discussion only and should not be relied on as legal advice.