Yep, that is actually a big problem (benefit?) with Wordpress, were all the code is redistributable because of the GPL license. So paid plugin developers are in this weird limbo were they are charging people for their plugins without telling them that the code is freely redistributable.

A plugin is a separate code module that wordpress loads, surely those are not bound to the wordpress core license. Extending wordpress with non-wordpress owned code does not grant them the right to push their license onto somebody else's completely unrelated code. If that were true, nobody would ever be able to sell a proprietary app on any linux distro. GitLab is open core yet extends the functionality with enterprise under a different, proprietary license.

There's no legal precedent yet, but WordPress plugins/themes are widely considered as being GPL because they're a "derivative" work.

Or, at least the PHP code in those plugins/themes are - possibly not the CSS + assets.

https://kinsta.com/learn/wordpress-gpl/