As usual blame payment processors (mainly MC/Visa). It wasn't always the case, in the 1980's there was plenty of incest porn movies (Taboo 1-4 and it's knockoffs, Taboo American Style, etc). The gov't didn't like it, but couldn't legally do much about it due to First Amendment, so got payment processors to do their dirty work back in the 1980's. Why they allow stepcest when it's an obvious work-around is still puzzling, but as usual forget trying to understand American politics and company decisions. Luckily in 2020's crypto is now a thing (can't follow the money, can't use it to censor anything, so fuck Visa/MC).