@lightweight
When it comes to plugins, plugins are almost always derivative works.
Wordpress thinks the same: https://wordpress.org/about/license/
If the pro/premium license is additional plugin functionality also under the GPLv2, but only provided if you pay, that's allowed.

If that's not the case, as wordpress plugins appear to be tightly integrated with wordpress, it's time to start GPLv2+ enforcement against such profit driven copyright violation - I'm happy to assist provided a listing of plugins to inspect.

@alilly
>Non-GPL Linux modules exist and one would expect them to be in a similar situation, yet the kernel has express technical functionality to support them - maybe look for the existing legal debates about that?
That's really an entirely different case.
In most cases, I would say that non-GPLv2 modules are copyright infringement (but sadly a lot of Linux copyright holders don't seem to enforce their license).

In Linux, there is an exception model known as EXPORT_MODULE_GPL() and EXPORT_MODULE().
Copyright holders can except the requirements of the GPLv2, but such exception only stands if such exception is agreed to by *all* relevant copyright holders.
You can do EXPORT_MODULE() to except the requirements of the GPLv2 under your copyright, but unless all other copyright holders that your code is a derivative work of have made an exceptions as well, the GPLv2 still stands and you doing EXPORT_MODULE() is misleading.
It's quite frankly ridiculous to say that you can make exceptions to copyrights that you don't hold.