I thought that the United States was a more tolerant country, but I see that I was wrong...
Tolerance have nothing to do with the problem. As
79flavors implied, it's more a question of perceived image than a question of effective Law.
Obviously, each party have to respect the Law, but as you demonstrated yourself, Law isn't something universal. While, because of the Spanish laws, you can access games with Incest while others can't, even just owning a game with zoophilia is a crime in your country (at least it was last time I looked at it), while it's perfectly legal in other countries, like the USA by example.
And this is the reason why each actor define his own rules around a moral consensus that go further than Law ; they are putting the limits where they estimate that "most of the public all around the world" want them to be.
It's, by example, why Patreon have rules more strict regarding their merchandising feature, than regarding their main activity. Mostly the public wouldn't care what game you install in your computer, but the same persons wouldn't necessarily agree if you were drinking your coffee at works in a mug with some explicit content from this game.
In top of that, payment processor have to deal with frauds, that they want to minimize since it's them which pay until the culprit is found, if he's found. And obviously, the darker is the topic, the higher is the number of frauds ; I'm sure that you understand that, by example, people buying p*d*shit rarely use their real identity, and so their real bank account.
Obviously, a game with a 14/16yo digital character isn't the same thing than real p*d*shit, but they share a small portion of their public, and therefore the risk of frauds are higher than with other kind of porn content.
All this being said, I don't agree regarding the lack of tolerance. The fact that all this isn't a matter of tolerance doesn't mean that Patreon isn't more tolerant than they should. They could easily actively enforce their rules. They could hire two/three persons to perform random check of the accounts content, or ask for a validation beforehand for any link published by an adult account, or keep an eye on any new creator account during the few months to see if the rules are effectively followed.
Instead they are waiting that someone report a violation of those rules. if you're reported, they have no other choice than to respect the rules decided by the payment processors, but until then, they decided to not care, up to you to stay under the radars.