It's one thing to choose how to license your own code; it's quite another to insist that others license their code the same way, simply because it may have some tenuous connection to your code.
I don't think the connection is tenuous. Is the theme intended to be run as part of Wordpess? Yes. Does the theme work without Wordpress? No. Sounds like a derivative work to me.
IMO there is a big difference between coding to an established interface (let's say POSIX) and writing an extension (theme/plugin/whatever) that is intended to run only as part of a specific piece of software (like Wordpress).
If you want to create your own non-GPL blogging software to run your own themes, go for it. If you want to save time by using someone else's work, you have to abide by their rules. I am pretty sure that this definition of derivative work has not been tested in court yet, but it really has nothing to do with the GPL specifically.