As I understand it, Wordpress allows designers to create packages of templates, styles and scripts that, taken together, configure Wordpress in a thematic way. These themes make the difference between site that looks like my Frontier News site and the memorial site for my father. My Droidie site has a Wordpress theme that makes it look like it is managed with Drupal.
This is not a new idea. Manila has themes. Tumblr does. Imho, any serious blogging tool not only gives the user the ability to customize the look and feel, but also offers itself as a platform to serious users who then package the configuration up so that other, less skilled users, can use them. (An aside, this is one of the things that's seriously missing from Twitter -- there's no way for designers to participate in its ecosystem.)
On one side are the authors of Wordpress, a GPL product, who say that these packages must also be GPL, because of the virality of the GPL. And on the other side are at least some of the theme developers who say they are new works, do not contain any of the GPL'd code, and therefore are not necessarily GPL.
Well, there's certainly a line in there somewhere. I think it's obvious, for example, that you can write a closed source app that runs on Linux. But the OS loads the app, it calls routines in the app when the user clicks a key or moves the mouse or pulls down a menu.
The program counter of the CPU crosses lines-of-license millions of times every second on the computer I'm using right now. The editor I'm using is GPL. The kernel of the OS is FreeBSD which is open source. Inbetween are layers of operating system that are closed source owned by Apple. We live in a heterocode world. In that way the problem of Wordpress is not unique.
And is a theme more like code or is it more like a document? That's another line. I mean suppose I use Photoshop to edit a graphic. Clearly since it's not open source, I can choose not to freely license the graphic I create. But suppose I use an open source graphics program? Am I obliged to share the graphic? Obviously not.
So in this case, it seems to me it's really up to the designers to choose if they want to share their work under the GPL or not. Sure, they've benefited from the work of open source developers, but then so have the developers of Wordpress benefited from the work of others, open source and not open source, without compensating them or reciprocating. It happens all the time. It's the way of the world, you might say.
Further I don't see the harm in it. If there's a market for what they do, why shouldn't the developers make some money, and why shouldn't users be able to pay for it, if they want to? It's not as if there are a lot of protections for this kind of work -- so it seems inevitable, practically speaking, that we get the benefit of their know-how even if technically the code isn't licensed under the GPL. If I can do a View-Source, does it really matter if it's GPL or not?