Remember that the GPL is a copyright license. This means that the only cases in which it matters are when the work is being distributed. The problem with trying to coerce plugin authors into using the GPL is that plugins are usually distributed separately from the core application. This means there's no legal justification for suing somebody who writes a proprietary plugin, because they're not participating in any action relevant to the GPL.
If I wrote an application which supports plugins, and I wanted to prevent a particular non-GPL plugin from being used, the only legally justified way I could do so would be to sue my users. That's right -- I'd have to attack the very people using my software. Not the plugin authors, the users.
Ironically, this means that selling a proprietary plugin is safer than hosting it. I wonder what Dries would think about that.
I believe the use case at hand is, given person Tom who has purchased a premium WordPress theme, is the theme a derivative work, allowing Tom to redistribute the theme to whomsoever he chooses?
If I wrote an application which supports plugins, and I wanted to prevent a particular non-GPL plugin from being used, the only legally justified way I could do so would be to sue my users. That's right -- I'd have to attack the very people using my software. Not the plugin authors, the users.
Ironically, this means that selling a proprietary plugin is safer than hosting it. I wonder what Dries would think about that.