A (long) while back, I created a library for WordPress that allows plugin developers to easily add new buttons to the editing area, regardless of whether an author was using the rich text editor or the plain text editor. I released the library to the community, and be necessity it is licensed under the GPL.
Along the way, others have used the library in their plugins. One of the people that has used the library is Oliver Seidel for his cforms plugin. Unfortunately, Oliver has come under fire recently because someone pointed out that his plugin is not GPL-licensed, and as such has rightfully been removed from the WordPress.org plugin repository.
I commented on King Rat's site that if the cforms plugin used my GPL library, then it must be released under GPL. In response to that comment, I have received the following email from the author of the cforms plugin:
I don't have an issue with users not showing my credit line underneath their forms, there are in fact 0-cost options to remove it. Even without loosing the ajax feature.
Whether or nor GPL has never been an issue to me, I in fact encouraged people to change the source code and gave tips and examples.
I simply didn't approve of the fact that people would use my work, remove the credits and resell to make profit.
Being a developer I think you understand.
With that said, I will remove your code immediately, per your wish.
If you don't approve of people removing the credits and selling your plugin, then you should really market your free plugin better, which would make it easier for users to choose your free option over someone's for-pay version of your work. Surely not releasing the plugin as GPL, therefore making it unavailable via the WordPress.org plugin repository, makes it harder for people to find you and your services. Face it, you're just shooting yourself in the foot -- the people who don't want to pay to remove your link are just going to remove it anyway. The people who want to redistribute your plugin (which doesn't have any license file in it, and so one would assume it's GPL based on it's use of GPL source and relationship with WordPress) are simply going to do it anyway, whether you think it's legal or not. You're just losing all of the incoming links via exclusion from the directory and bad publicity because of your decision.
Regardless, having reviewed the plugin, there are other snippets of code that are surely derivatives of existing GPL code. It would be very difficult, even if the buttonsnap library was removed, to say that the plugin used no other GPL code. I'm sure that the author will disagree, but it would be much simpler to properly license the plugin as GPL, put it back in the directory, keep all of the extant source, and reap the benefits.
This is one of those weird cases where giving your code away is just more profitable than restricting access to it.