Back in 2010 Mark Jaquith posted an excellent article on "Why WordPress Themes are a derivate of WordPress" in his blog "Mark on WordPress".
Mark gives an in depth-analysis of how WordPress and Themes interact and sums up th GPL related arguments from Matt Mullenweg's discussion with Chris Pearson on the licensing of Thesis.
His key findings are:
"Theme code necessarily derives from WordPress and thus must be licensed under the GPL if it is distributed."
"Themes interact with WordPress (and WordPress with themes) the exact same way that WordPress interacts with itself."
Jaquith also outlines that the same principles apply to the other highly popular Blogging Platforms Joomla! and Drupal as well - they are both released und the GNU Public License as well:
"Drupal modules and themes are a derivative work of Drupal. If you distribute them, you must do so under the terms of the GPL version 2 or later."
Read the full article "Why WordPress Themes are a derivate of WordPress".