Ideally it would be like that, but keep in mind that the GPL license is not a law. The developer can choose not to publish under GPL, and if Wordpress does not like it, then they can delist the plugin on their website. If the developer included code in his plugin which was distributed under GPL (for example by Wordpress Foundation), then the original creator/rights holder might have a claim against the developer IF conditions are met and this constitutes a violation of the license in any way. Still it is too risky for the end user to derive a right from this to be allowed to violate the copy rights of the plugin developer. And another important aspect is that calling a WP function like "add_action()" does not automatically turn the plugin into a derived work which might have to adapt or follow a certain license. So, while it would be ideal in the eyes of Wordpress if all plugins and themes were publicly published under GPL, it does mean that all will be published in that way and it does not allow the end user to conclude that plugin/theme is actually published under GPL. When I give you a plugin and you find a license.txt with GPLv3 in there, how do you know that I did not just place it there? If you would then use the plugin in good faith, you would still be liable towards the rights holder while you would also have a claim against me (but I might be gone already or might not have any funds to satisfy your claim/damages).