There's no legal precedent that a theme is somehow a derivative work of the software it's designed for. There haven't even been any good cases to give precedent for dynamic linking creating a derivative work. That's the only way the GPL could possibly apply, since nobody's distributing WordPress with their themes.

How is taking the HTML for a webpage you've created, replacing the title with <?php the_title(); ?> and putting <?php the_content(); ?> in the content area, a derivative work of the WordPress code base? Yet that's the argument they're making when someone say you're bound by WordPress's license by creating a theme that will work with it.

You can't just throw a license at someone that hasn't agreed to it. It's a completely untested legal theory Matt relies on when he says WP themes have to be GPL. There are lots of people that don't buy it at all, like the creator of Thesis, which is one of the best selling commercial WP themes.