Though I'm not familiar with Wordpress and it's theming system, this argument has indeed puzzled me. This article is spot on: why would a theme be beholden to the software's license? That said, maybe Wordpress themes are much different than normal themes, and are much more tightly coupled? That WP would pursue this if not puzzles me.
> Why would a theme be beholden to the software's license?
Well, because there is WP PHP code in it. Once you do that you become a "derivative" and you take the license.