Gumroad used Stripe, the same payment processor who abandoned Wishtender which was due to Mastercard and Visa pressure...
Marketplaces have the power to change processors. Ebay demonstrated this recently by dropping American Express due to their high fees.
Processors also face increased competition from competitors like Paypal and Bitcoin. Remarkably, Paypal manages to partner with - and compete against - Visa and MC.
VISA and MC may indeed be guilty of using their clout to force media marketplaces to filter their content.
But, as I've stated, if a marketplace allows a processor to filter their content, it is a business decision. They have the freedom to change processors.