There are many, many companies making millions per year selling themes and plugins for WordPress, even those that acknowledge they're GPL licensed. Those that want to make a deal of it put in license terms that while their code is GPL, the other assets necessary to use that code -- such as stylesheets, images and JavaScript files -- are commercially licensed.

Those that don't just don't mention it, like WooThemes. They're one of those millions-a-year businesses. You won't find GPL or any license terms mentioned on their site or during checkout unless seek out their "Terms and Conditions" page yourself -- a link they hide as much as possible -- black text that doesn't look like a link under the logo in the footer, apart from even the other footer links.

In reality, the licensing issue isn't a big deal. There are over 50 million active WordPress installs -- it's a huge market. If you provide an awesome plugin, and you support it, people will pay you for it, even if they could download it off some warez site and you'd have no recourse to go after them.

Is this actually true? Wordpress is GPL, but why would a plugin need to be under GPL, unless it used actual code from Wordpress (not just API calls). Can someone clarify?

http://wordpress.org/about/license/:

Part of this license outlines requirements for derivative works, such as plugins or themes. Derivatives of WordPress code inherit the GPL license. Drupal, which has the same GPL license as WordPress, has an excellent page on licensing as it applies to themes and modules (their word for plugins).

There is some legal grey area regarding what is considered a derivative work, but we feel strongly that plugins and themes are derivative work and thus inherit the GPL license. If you disagree, you might want to consider a non-GPL platform such as Serendipity (BSD license) or Habari (Apache license) instead.

There could still be a service opportunity there. If it's especially easy to use his plugins via his service than there could be a business.