All Wordpress Themes Are GPL, Too

As far as I can tell, SFLC is operating under the theory that if you write something that is not useful except when used in conjunction with X, and when executed is somehow combined with X, and your thing makes calls to functions in X, your thing must be a derivative work of X, and so you need copyright permission from X's copyright owner.

I don't see any way to reconcile that approach with the case law. For example, there have been attempts to use copyright to stop people from making unauthorized games for video game consoles. Note that a console game is not useful except when used with the console, when executed it is combined with the console code (the classic consoles, at least, generally did not have any memory protection or distinction between system code and user code), and the game makes calls into the console system software. The unauthorized game makers generally won these suits.

The question that needs to be asked when you write a theme or plugin is do you have to actually incorporate any copyrighted elements of WordPress into your source code? (Note: the case law supports the position that if a it is necessary to incorporate a copyrighted element in order to interface to something, then that is OK...the copyright monopoly is supposed to be a limited monopoly, and cannot be used to get a patent-like monopoly on something by controlling the interfaces).

So, for example, if someone were to reverse engineer WordPress themes and plugins, and write and publish a document on how to write theme, but were to not include copyrighted elements from WordPress (other than those necessary for interfacing), then you could sit down and use that documentation to write themes and plugins, and my contention is that you would not have created a derivative work, and you could go ahead and distribute it to users who could use it with their WordPress installations.