Well that's just Steam, I have no idea how Kagura Games manages to get the game on Steam maybe because they had the customized version from the beginning and you can just download the +18 patch for free from the Kagura Games website as a player.
This doesn't seem to be a problem on Steam.
For a few years now you can only install and play adult games/DLC via Steam Key on Steam if you live in Germany for example.
Thanks for the answer though.