Free companies, hooray
I find it disturbing that so many people will speak up for poor little Mastercard and Visa and their "right" to do whatever they want. These are the same companies that will let you pay for hookers, booze, automatic weapons, whatever, with your credit card. They are using their immense market power to assist the US in a lawless crusade to crush a political movement.
Assange appears to be a preening showpony. This does not detract from the extremely alarming notion that a government and corporate alliance is being used to assault an organisation comprised of private citizens.
I don't know about your home countries, but here in Australia paying by Visa or Mastercard is often the only option presented. So it is not possible for people to remain fully involved in society but also have nothing at all to do with these companies.
All legal niceties aside, this conduct is disgraceful and should be condemned.