Yeah, you like that rhyming! Don't lie, it's catchy. Onto something meaningful:
I've spent a lot of time doing WordPress work. I've spent some time in the community and a lot of time using, hacking, and theming WordPress. As I've stated before in other posts, this site is powered by WordPress. Being so involved with WP, I decided to pay attention to an important debate that I saw roll across Slashdot: Should all WP themes and plugins be GLP'd?
Sure, the GPL's viral...I get that. The Fearless Leader of Wordpress argues that themes can't run without WordPress, therefore they are derivative works. Others argue otherwise. So the fundamental questions are, "What is a derivative work?" and "Is a WordPress theme a derivative work?" On the first question, the GPL v2 (the license that WP uses) has this to say:
0. This License applies to any program or other work which contains a notice placed by the copyright holder saying it may be distributed under the terms of this General Public License. The "Program", below, refers to any such program or work, and a "work based on the Program" means either the Program or any derivative work under copyright law: that is to say, a work containing the Program or a portion of it, either verbatim or with modifications and/or translated into another language. (Hereinafter, translation is included without limitation in the term "modification".) Each licensee is addressed as "you".
Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does.
If you're theme contains a portion of WP, then it must be GPL'd. So, what constitutes "a portion?" All WP themes contain API calls -- does this mean that it's a derivative work? Several people argue both ways, but I'm going to choose a side here: merely using public API calls does not make your work a derivative. Why not? Well, look at the definition of "work based on the Program" in the quote above. No modifications are actually taking place. In the case of a theme, I can look at the first line in the second paragraph ("Activities other than..."). If your theme does not contain a copy, is not distributed with, and does not modify WordPress (in the source code sense), then the license doesn't apply.
Here's fun one. Matt Mullenweg made this argument during a verbal exchange:
I think just one way to test it is, you know, take a screenshot of a website running WordPress without Thesis and then take a screenshot of a website running Thesis without WordPress.
I'm not going to get into specifics with Thesis. Let's, for the sake of argument, substitute "Thesis" with any theme in the world. Here, Matt is appealing to a common-sense definition of a "derivative work." It makes sense and is a very convincing argument. I, however, reject this argument because the GPL (poorly) defines "derivative work." If I didn't touch your source, it's not derivative. Sure, my theme might be designed for WP, but that's not a derivative as per the license.
Matt's argument also appeals to the idea of respecting the Open Source Community: if I release a theme under a commercial license, am I some how ripping off the individuals that contributed code to the project? My answer to this is no. One of the ways that Matt (his company, Automattic) makes money is by performing spam filtering for WP blogs. It's called Askimet. In fact, he mentions this during the verbal exchange that I linked to earlier in this post. The Askimet plugin for WP, presumably GPL (I didn't go check), provides access to Askimet's spam filtering service. Ok, cool -- the plugin is GPL (presumably), but the service isn't open-source. I can't go grab the Askimet service's source code and host my own spam filtering. So what? Well, if I wrote a plugin that preformed the spam filtering WP-Side (on the host that is actually running WP), Matt would (presumably) argue that this rips off the community and is a violation of the GPL. Clearly, he's ok with moving the functionality elsewhere and then charging for it. Really, he's only GPL'd the interface between WP and Askimet. Is this really open-source? Is this ripping off the open source community any more or less than a closed-source plugin that performs this same function WP-side?
I don't think so. What's the difference? In both cases, you're preventing the user from tweaking the functionality (at least to some degree) of the service. The Askimet plugin might as well be closed-source -- most of the magic happens behind curtains that I can't see through. In the end, the net effect is the same.
Besides, who can get mad at someone adding value to WP though closed-source means? Sure, it'd be more beneficial to the community if all plugins, themes, etc were OSS, but closed-source software can still add value, too. As long as no code is stolen from WP, they shouldn't care. Closed source is not the devil. Perhaps I don't understand the WP community enough to really talk too much about the "rip off" factor.
I don't know -- this doesn't directly effect me at the moment, but this issue still makes me a bit upset. It's frustrating that some people try to make the GPL infinitely viral. I love open source software, but I think that forcing me to open-source any themes that I create is taking it too far. I work on an open source project (GPL v2), and I can't say that I'd get upset at someone developing a closed-source add-on and making money off of it.