Originally Posted by AdultKing

The GPL is not forced on anyone. People are free to release software as GPL or under their own proprietary license.

It is true that if you develop a derivative work of Wordpress or any GPL software for that matter that you should inherit the license. However Wordpress does provide some scope to use the Serendipity or Habari license if a developer chooses.

Nobody is holding a gun to the head of developers, they can elect not to release derivative works of Wordpress which is GNU/GPL licensed itself or they can choose an alternative license.

Some developers of some Wordpress plugins which are protected proprietary works achieve this by separating the Wordpress derivative logic from the proprietary logic and then release the derivative logic under GPL and the proprietary logic under their own license.