This is all about laws, publicity and association. Patreon and PayPal are based on US laws and those can be quite prudish when it comes to certain fetishes. When a company wants to extend their business to families, desire the support of certain (prudish) magnates or politicians, or are simply forced to because of their laws, they can't be seen dealing or supporting "bad stuff" because it will hurt their reputation.
The costumer isn't at risk mostly because law enforcement can't go after everyone (also because not everyone who supports "bad porn" is on US territory) so they decide to go to the root of the problem instead. Patreon. And because Patreon doesn't want the men-in-black knocking at their door, they crack down on anyone using their platform to support "bad businesses". To use your analogy, it's like catching the cashier (Patreon) supporting potentially illicit activities in the basement on their establishment. Or Toys R' Us selling cannabis to kids, hiding the stuff inside the toys.
Anyway that's the gist of it. Someone can probably explain it better than I.
TL;DR: It's less about where YOU spend your money on and more about a big company providing fringe support to activities that the law, certain markets and financiers don't want to be associated with or are entirely against.