For moralizing activist reasons. It's nothing to do with legality. With any luck eventually they'll inadvertently trample a sacred cow of whichever party is currently in power and we'll finally get sane legislation outlawing their overbearing nonsense.
I doubt it's moralizing reasons, it's probably because once you disseminate porn, you become a vehicle for child porn, which is a legal and PR disaster.
I believe some of these sex trafficking laws implicated a broad sweep in their litigation and Visa/Mastercard doesn't want to have to go to court over a these things.
If you disseminate user uploaded porn then moralizing activists can certainly accuse you of that. It's performative hand wringing though. The admins assuredly don't want to distribute it and the goal of anyone publicly uploading that on the clearnet is to harass and disrupt rather than to disseminate.
Anyway "child porn" as well as the broader "legal reasons" fails to explain the US payment processors' moves to block all sorts of content and products over the years. Even including porn that isn't user uploaded (and thus has proper records keeping).
There are all sorts of adjacent reasons too, like human trafficking and prostitution. It's probably similar reasons to why landlords are so aggressive about renters not doing any sex work, because even indirectly receiving proceeds from prostitution is illegal, even when you're unaware of it.
In which jurisdictions? I'm skeptical. If a vendor takes reasonable precautions it should not generally be possible to hold them liable.
Anyway you've circled back to user generated content. But again, that's far from the only thing that payment processors have discriminated against over the past couple of decades.
For moralizing activist reasons. It's nothing to do with legality. With any luck eventually they'll inadvertently trample a sacred cow of whichever party is currently in power and we'll finally get sane legislation outlawing their overbearing nonsense.