Well, in my view, the primary goal of the GPL is to retain the freedom of tinkering to avoid dead-ends and to avoid someone building on your free software to take advantage of you. Imagine you write a "green_grass" module and publish on drupal.org. Someone takes your module and writes a "greener_grass" module and sells it on their website for a nice sum of money. Well, the GPL is supposed to protect you from this to happen, if the new module is derivative of your work. They'd need to distribute greener_grass free of charge and should only ask for money up to the cost incurred for transferring the source (which given how small Drupal modules mostly are, is probably a couple cents).
Now what counts as derivative work? In my example, green_grass serves as base for greener_grass. But do both serve as derivative works of Drupal and therefore fall under the same requirement? As far as I understand, they do. The same thinking applies to not just Wordpress plugins but themes even, for the source code at least that interferes with Wordpress, and the recent Thesis theme hoopla proves that: http://mixergy.com/chris-pearson-matt-mullenweg/
The application of this to Drupal themes is clear. If you look closely, you'll see that most if not all Drupal theme marketplaces build a base theme and publish on drupal.org free of charge. This makes people use their base themes, get bug reports and fixes, etc, but that code is GPL bound anyway, so why not publish? They do sell Drupal themes though, due to themes having components which are not derivative to Drupal, not source code running in Drupal's environment. Namely images and styles. The rest, the base theme is on drupal.org to tinker with.
Not many Drupal modules exist, which do not work with Drupal via the theme, node, menu, etc. systems, and if there would be such modules, they'd be so independent of how people used to work with Drupal (eg. customization via standard themes, editing on the node UI, building views based on the data, etc.), that the community would find little value in them. So finding ways to free modules off of the rules of the GPL sounds pretty far fetched.
Yes, there are for profit components, base themes, etc. for Wordpress and Joomla. I think Matt Mullenweg sent a pretty clear message with the Thesis case on Automattic's stand. As far as I see, the Drupal community was always true to that thinking. Yes, the GPL is written to protect developer and software freedoms and has few considerations for business, or when it does it protects the developer and software freedoms vs. business.
More information about formatting options