Indeed, you're right about creators who don't want their art used in porn content even if I'm more familiar with that regarding non copyrighted music.To give my opinion about this, I spoke with a creator in the KK community once, mainly because I wanted to use his work to enhance a scene in my game. He was actually very happy to know that I was interested in his material, and was totally fine with me using it.
The KK creators actually wants their scenes to be used. That's why they are sharing them with the community...
That being said, I don't know if Shli asked permission before using someone else's work, but if he didn't, I would recommend asking next time. Some people might be against sharing their scene for something related to an adult game. Or be against the themes in your game. Of course, that's just my personal opinion; there's no hard rule here as far as I know...
KK being an ero game at first is less prone to that but asking is the right thing to do yeah. I should pay more attention to that in the future.