I think Illusion's more or less come out and said they don't really care about people making games with it, or implied it at the very least, I don't recall where I saw it though.
More precisely, they said that you have the right to use the images generated by their studio software, as long as you keep the copyright notice that come with it. It was in a 3-4 years old thread regarding their US distributor, who don't apply the same rules.
They have the right to be stricter than Illusion, but for them to win the case, they need to prove that you are using their version. Paradoxically, showing that you use a pirated Japanese version, with possibly a fan made translation, should make you win... and, of course, make them sue you again, this time as Illusion representative and for piracy.
The problem is Steam and their IP rules, if I recall, which is why you only see them on Patreon and Itch (though maybe only on Patreon, depending on where Itch goes.).
Technically Steam would have nothing to say
if you use a legally owned Japanese version
and keep the copyright notice in all the images. At least unless Illusion changed their rules since their 3-4 years old statement. But most games remove the copyright notice and use a pirated version.
Still technically, Patreon and Itch.io should do the same than Steam, banning you unless you keep the copyright notice and can prove that you have bough the game.
But in the end, what happen is mostly that only Steam care about those games, for a question of reputation more than anything else.
Illusion don't care much what happen outside of Japan. Their US distributor (and only distributor as far as I know) would spent more suing you, than they could expect by wining. As for Patreon and Itch.io, they have a lesser need for a pure reputation than Steam, so they let you earn fuck bucks and take their share ; what probably pay one or two employees, probably not much more.