Fan (porn) games in general aren't immune to this. For every game trying to make money out there that licenses assets the copyfriendly way, there're the free-as-in-beer games that just dump screencaps from other H-games into the backdrops. When you're a hobbyist, make no money from your work, and have pretty much no resources for game development, I imagine it's pretty tempting to do this. It's especially so for something as critical-but-unimportant as background art.
Heck, as far as that goes, I think even for-profit Japanese developers do a ton of this. I dunno if they use the asset studio model or just directly work with artists and VAs like contractors. But you can open the readme of pretty much any doujin game and find one or seven links to independent content creators or free content websites at the bottom. I'm not trying to say they aren't doing anything legit. Their small-group porn game development industry just has a lot more access to resources than the West does, which is probably one big reason why there're a lot more not-really-professional porn games coming from there.
That's ANOTHER thing that a major adult game development platform would want to be: a place for those kinds of independent workers to connect with small developers. Kinda like the Unity asset store, but for drawn dicks and moaning college freshman VAs.
You know, as far as that goes, assets licensed through something like the Unity asset store are probably fair game for use in content not made in Unity by the licensee. Although it wouldn't surprise me if they attempted to keep that market captive by some licensing provision disallowing content developers from using Unity store assets in projects not developed in Unity games. Never actually looked that far into the license for it. The people who indie-dev for money know a whole lot more about it than I do 'cause they kinda have to.
