I don't know of any industry where a piece of IP which receives no further royalties and holds no future market value is deliberately kept internal other than cases where the release/proliferation of such information would incur costs.
Seriously? This happens all the time, especially in the games industry. Studios get bought and closed down and their IPs RIP into the aethers, because the new owner corp has too big a dick to swing around, but also pockets too deep to bother with the relatively small IP themselves.
Then there's also big corps like disney with a gigantic backlog of IPs, because they just blanket copyright anything and everything that could be loosely associated with the working title of their new secret project.
People aren't asking for dead game creators to send them the renpy sdk files with all images, they merely want to know the resources used so they can create characters and story in the same universe.
But the point of contention was that "AVN culture" is somehow supposed to be uniquely protectionist. Compare the AVN scene to what nintendo would do, if you released something in the pokemon universe, though.
It's more analogous to if a chef abrubtly quit the business and was unwilling to share his recipes, even though it was absolutely certain those recipes held no specific market value for him. Or a musician unwilling to mention which model guitar and amp he used for a specific track. Are they in the legal right, absolutely. Do they come off as hyper-protectionist and misanthropic, yes.
I mean, I agree with you. AVN devs certainly aren't over-relying on Daz for artistic integrity. They probably aren't even in their legal right to protect their assets, which a majority of them just leave stock (like this vampire girl that's in every second non-vampire AVN of the last year or so, where most of the time they don't even bother removing her fangs).
But still the question is what makes AVN culture hyper-protectionist? I don't think a few grumpy pirates going "Hey, I know that landlady from that other game." somehow counts as protection.