UPDATE: Please also read the follow-up post here.
The GPL v2 has the following clause in it (right at the top – emphasis mine):
1. You may copy and distribute verbatim copies of the Program’s source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and give any other recipients of the Program a copy of this License along with the Program.
The GPL v3 has a similar clause (again, emphasis mine):
4. Conveying Verbatim Copies.
You may convey verbatim copies of the Program’s source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice; keep intact all notices stating that this License and any non-permissive terms added in accord with section 7 apply to the code; keep intact all notices of the absence of any warranty; and give all recipients a copy of this License along with the Program.
You may charge any price or no price for each copy that you convey, and you may offer support or warranty protection for a fee.
While there is absolutely nothing wrong with charging for GPL’ed themes (not making them available for free), if I receive a theme under the GPL I can then redistribute it (for free or for a fee) as a right granted by the GPL.
So I can then choose to set up site where I make all of the GPL licensed themes that I purchased available for free download, under the terms of the GPL.
The GPL also allows for derivative works.
2. You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions: [...]
So I can make a few changes to the themes (for example, port parts of them to the Carrington framework for easier development and modification), and distribute those as well.
Yay for software freedoms!
I purchased all of the WooThemes and StudioPress themes this morning. Which should I do first, set up free distribution of these GPL themes or start integrating the Carrington CMS theme framework into them?