Some of the most intense debates I've seen in the open source community have been discussions covering what is or what isn't required by the popular open source license, the GNU GPL. For example, it is common practice in the open source CMS market to distribute themes that include the templates (the code) under the GPL and the artwork (including CSS) under a different license. Under the GPL, is this practice legal or not?
On opposite ends of the GPL debate are staunch supporters for and against the GPL. Someone usually argues that distributed theme packages for a GPL licensed CMS must always be distributed under the GPL. To do anything else would be illegal. Another person will usually argue that the time and money the theme developer put into a theme gives him or her the right to distribute the theme any way they wish. When the discussion has finally reached this level of completely opposing views, the sparks will fly.
This time around it was the Wordpress community that found itself asking the various legal questions. Matt Mullenweg of Wordpress asked the Software Freedom Law Center for some legal clarification on the GPL issue. Today, Matt posted an article with the law center's response and provides his own input too. The truth can be found somewhere in the middle.
If WordPress were a country, our Bill of Rights would be the GPL because it protects our core freedoms. We’ve always done our best to keep WordPress.org clean and only promote things that are completely compatible and legal with WordPress’s license. There have been some questions in the community about whether the GPL applies to themes like we’ve always assumed. To help clarify this point, I reached out to the Software Freedom Law Center, the world’s preeminent experts on the GPL, which spent time with WordPress’s code, community, and provided us with an official legal opinion. One sentence summary: PHP in WordPress themes must be GPL, artwork and CSS may be but are not required.
You can see Matt's post which includes the letter from Software Freedom Law Center on the Wordpress Blog.
It's good to point out that while people and commercial companies are able to distribute and promote GPL themes with non-GPL artwork, a number of open source projects won't do so. For example, Wordpress.org will only promote commercially supported themes if they are distributed 100% under GPL, including artwork and CSS.
The reasons for not hosting anything but GPL projects varies from project to project. However, I think one of the better reasons for allowing only GPL files through the door is to keep it simple.
Open source projects distributing software have a difficult enough time managing and abiding by the terms of their license. When you throw in a varying number of additional licenses for the same software package, such as a Wordpress theme, the burden for any project to abide by all those licenses significantly increases. The last thing open source volunteers want for their project are additional burdens.
Note: This story was revised from an earlier version for expansion and clarification. Also, credit to Mike Johnston, CMS Critic, for getting parts of this story out first.