But the hypothetical proves that the theme is NOT linked with Wordpress code when it is distributed. The linking is performed by the end user when he "adds" it to his Wordpress system.
The difference is WHEN the linking occurs. In PHP and most interpreted languages, the linking occurs at runtime.
So if you distribute Theme unlinked, you can argue the GPL does not apply since it has not yet been linked with GPL code. In fact, you never know if WP changes it's license to NON GPL, or someone creates a WP clone with the same theme APIs which can run the same themes. If this were to happen then it becomes obvious the Theme does not have to be made GPL.
It's not a clear cut case that the GPL can be applied to themes. Themes are not like C programs that do early binding. Once a C program links to a GPL library it only works with those GPL libraries.
PHP files late bind. The program calling the library does not get locked into the library until runtime.
Here's a way to demonstrate how late binding can complicate the issue. Imagine someone implemented a mock version of all the WP library functions the theme used and those mock libraries were not GPL licensed. Combining the theme with those mocks would not force the Theme to be GPL.
So, I think it's valid to argue that Theme files only have to be licensed under the GPL once they get combined in a word press install. The theme developer cannot assume to know what the end user will combine the theme with. Perhaps it will be with those mock libraries.:)
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But the hypothetical proves that the theme is NOT linked with Wordpress code when it is distributed. The linking is performed by the end user when he "adds" it to his Wordpress system.
The difference is WHEN the linking occurs. In PHP and most interpreted languages, the linking occurs at runtime.
So if you distribute Theme unlinked, you can argue the GPL does not apply since it has not yet been linked with GPL code. In fact, you never know if WP changes it's license to NON GPL, or someone creates a WP clone with the same theme APIs which can run the same themes. If this were to happen then it becomes obvious the Theme does not have to be made GPL.
PHP files late bind. The program calling the library does not get locked into the library until runtime.
Here's a way to demonstrate how late binding can complicate the issue. Imagine someone implemented a mock version of all the WP library functions the theme used and those mock libraries were not GPL licensed. Combining the theme with those mocks would not force the Theme to be GPL.
So, I think it's valid to argue that Theme files only have to be licensed under the GPL once they get combined in a word press install. The theme developer cannot assume to know what the end user will combine the theme with. Perhaps it will be with those mock libraries. :)