The government really did screw over websites like Craigslist and Backpage, but that was due to changes which would have made the site operators responsible for the content their users post which I don't think would have an impact on someone like mastercard.

I suspect the payment processors must support the anti-porn agenda since I doubt they really fear that any appreciable number of people will resort to using cash only for the rest of their lives in protest. It's entirely within Visa's power to say "If you don't like porn don't pay for it, but it isn't our place to police what free people can do with their money. Our role is simply to facilitate any legal transaction. If you feel something shouldn't be legal speak to your representatives and change the laws"