Doujin works are made with the awareness that they are parodies of the original work. It does not alter the body of the original work in any way and, as the term itself means, self-published. It is made without any direct affilation in regards to the original work.
I've noticed spillover effects into Japanese gamers as well -- people being suspicious of or derisive about mods, even when they're perfectly legal and the game has built-in mod support (looking at you Monster Hunter World).
My (Japanese) girlfriend is on the very conservative side of the spectrum there and absolutely hates it when I bring up any kind of modding, and so do her friends -- the culture of "authorial intent is king" is very strangely strong for a culture that also appreciates and enjoys doujin.