Context matters: It's a very different dynamic, depending on who approaches whom.
If the venue owner does the approaching (as in the context of the post raising this sub-thread) like Apple, Microsoft or Google approaching extension developers) it's questionable.
If the musician (or the extensions developer) approaches the venue owner, it's an entirely different story.
One has exploitation written all over it, the other not so much.
The context of the great-great-...-parent post suggests the exploitative version.
If the venue owner does the approaching (as in the context of the post raising this sub-thread) like Apple, Microsoft or Google approaching extension developers) it's questionable.
If the musician (or the extensions developer) approaches the venue owner, it's an entirely different story.
One has exploitation written all over it, the other not so much.
The context of the great-great-...-parent post suggests the exploitative version.