When Night Falls

"Mastercard has not barred any legal transaction" is, technically, a true statement – because Mastercard is not the one processing the transactions in the first place. Mastercard does not deal directly with any merchant, and in fact typically refuses to communicate with merchants at all; there's always a payment processing service sitting in between Mastercard and the merchant, whether that's Stripe or Paypal or any of dozens of other service providers.

Consequently, there are two layers of service agreements in place: the agreement between Mastercard and the payment processing service, and the agreement between the payment processing service and the merchant. That second layer of service agreements, between the payment processing service and the merchant, is where all of these content restrictions are being imposed. Mastercard can thus truthfully claim that they aren't barring legal transactions.

Now, if you've been paying attention, you've probably already spotted the issue: if the content restrictions are being imposed upon the merchants by individual payment processing services and not by Mastercard, why do all of those payment processing services seem to have exactly the same content restrictions?

That's where the critical sleight of hand comes in: while Mastercard's own terms of service do not require payment processing services to bar transactions of particular types, their ToS does require payment processors to bar transactions which could be damaging to the Mastercard brand. What constitutes damage to the Mastercard brand is not defined; it means whatever Mastercard wants it to mean. The payment processing services are thus in a position where they can be held in breach of Mastercard's terms of service for basically any reason, which gives them a strong incentive not to test any boundaries.