The author is mad at Stripe and PayPal for banning transactions involving unicorn wieners but this is imposed on them by the backing banks. The reason behind banning adult materials has to do with Puritanism and with the high rates of refunds on adult websites.
I have an issue with Stripe or Paypal banning merchants without recourse that do not sell adult stuff or anything else bad/high refund, just because 'AI flagged the account'. And I know from Paypal that they have used 'AI' (statistics) to flag accounts, without recourse, almost since they started. It did cost us a lot of money; we had no refunds or anything and paypal was simply (in email and on the phone) repeating…
Momentum. They are the big games in town because so many people use them, so many people use them because they are the big games in town. There was a time for both when they didn't suck as much as they do now, at least relative to what other options existed.