Illegal to load plugins with GPL licence, in music programs that isn't open source

[mystran]

Also, in case you didnt know how plugins work, its not Steinberg who initiate the loading of a plugin, its the plugin user. Its not illegal to provide a mechanism for loading plugins that conform to an API you wrote.
So, since this pertains to copyright law, the resolution of 'illegal' would be that the creator of the plugin would be responsible for taking an infracting end-user to court for breach of copyright by the act of loading code that only worked as a plugin because of use of a GPL'd API into an application designed to host plugins using that API.
Im not sure many plugin developers would want to win that case, and Im not sure they'd keep their license to derive from Steinberg's code in the first place.

This seems extremely important to me, otherwise a user could just LD_PRELOAD some GPL'd "plugin" to *any* application and cause an infraction. Which is obviously stupid AF.

AFAICT the terms of the GPL in the VST API only impact the distribution of Steinberg's software, and are selectively enforceable by Steinberg. They get to decide who is violating their copyright. They are not responsible for enforcing or even knowing about the copyrights of users of their API that develop other software.

From GPL v3:

2. Basic Permissions.
All rights granted under this License are granted for the term of copyright on the Program, and are irrevocable provided the stated conditions are met. This License explicitly affirms your unlimited permission to run the unmodified Program. The output from running a covered work is covered by this License only if the output, given its content, constitutes a covered work. This License acknowledges your rights of fair use or other equivalent, as provided by copyright law.

You may make, run and propagate covered works that you do not convey, without conditions so long as your license otherwise remains in force. You may convey covered works to others for the sole purpose of having them make modifications exclusively for you, or provide you with facilities for running those works, provided that you comply with the terms of this License in conveying all material for which you do not control copyright. Those thus making or running the covered works for you must do so exclusively on your behalf, under your direction and control, on terms that prohibit them from making any copies of your copyrighted material outside their relationship with you.

Conveying under any other circumstances is permitted solely under the conditions stated below. Sublicensing is not allowed; section 10 makes it unnecessary.
[...]
9. Acceptance Not Required for Having Copies.
You are not required to accept this License in order to receive or run a copy of the Program. Ancillary propagation of a covered work occurring solely as a consequence of using peer-to-peer transmission to receive a copy likewise does not require acceptance. However, nothing other than this License grants you permission to propagate or modify any covered work. These actions infringe copyright if you do not accept this License. Therefore, by modifying or propagating a covered work, you indicate your acceptance of this License to do so.

So section 9 (together with section 2) of GPL really makes it pretty clear that it's not an "EULA" and as an end-user you can basically do whatever you please as long as you don't distribute modified versions. I'm not a lawyer and this is not legal advice, but the way I'd interpret section 2 is that it's probably even fine for a company to have it's employees internally modify GPL software for company internal purposes without having to worry about it, so long as these modified versions are just for internal use.

This is really the essence of "copyleft" licenses in general: rather than protecting the rights of the "copyright holder" it's about protecting the rights of users to do basically whatever they please, with only the restriction that these rights (and the source code to facilitate exercising them) must be also granted to the users of any modified or derived works.