I think the point with GPL Joost is hitting on is very important.

It is currently not possible to create WordPress plugins without committing to the GPLv2 or higher license. The license states in §2:

You may modify your copy or copies of the Program or any portion of it, thus forming a work based on the Program, [… ] provided that you also meet all of these conditions: […] b) You must cause any work that you distribute or publish, that in whole or in part contains or is derived from the Program or any part thereof, to be licensed as a whole at no charge to all third parties under the terms of this License.

I believe it is out of question, that the source code of a plugin is „derived from the Program“, as in derived from WordPress.

Based on this assumption it is plain and simple not legal to publish plugins and themes, which source code is not licensed as GPLv2. There seem to be questionable loopholes, that this could not apply to styles, but generally: If it does not do GPLv2 in part, it’s not legal.