Wikileaks was engaged in a violation of that democratic country's democratically created laws against the distribution of classified information.

The rule of law goes both ways.

It's even worse than that. Not only can MasterCard arbitrarily refuse to accept payments for a business, it can cancel any consumer's account with little recourse. Let's say that someone steals my credit card number and uses it in a foreign country, and MasterCard detects this fraudulent use of my card and shuts down my account. At that point, if I can't buy food and other essentials with cash, I'd be in real trouble. Also, it's hard to get a credit or debit card if you're unemployed - if you have no income and/or no credit history, credit card companies and banks don't really want to do business with you. And if you're homeless and begging for money on the street, nobody is going to lend you their credit card.

Of course you'd have to get a debit card - just with zero credit.

all of your consumer-side complaints are about banks not MasterCard.

Since the banks issue cards on behalf of MasterCard, they essentially constitute a single, tightly-coupled system from the point of view of the consumer. And the problems I described exist no matter which part of this system manages the end-users' accounts.

that's incorrect. in practice, MasterCard cannot do any of the things you suggest, aside from "arbitrarily refuse to accept payments for a business", which they have already done. your bank could shut down your card due to fraud, which may suck for you, but you can switch banks if you disagree with them. unemployed people can absolutely get visa/mastercard debit cards. ditto homeless people.

mastercard deserves criticism, but try to actually stick to things under their control (e.g. their appalling but now discontinued ban on wikileaks donations).

How is that different from freezing someone's assets?

It was sort of under the table, not done via regular, publicly visible means. It's prone to abuse. Asset freezing usually (always?) comes from a court order, which is generally done in public, with arguments from both sides.

Often it is not. Funds are frozen without any due process whatsoever, or even giving the account holder a specific reason. I know of instances where tax authorities have simply seized accounts on their own authority without sanction from a count. I know of instances where real estate has been seized on the mere suspicion of Marijuana growing. Yes, due process is available to remedy such wrongs... by suing in court for relief. Good luck with that if you have no liquid funds to hire a lawyer. And your bank is foreclosing because you can't pay the mortgage.

More and more, KYC means asking permission to spend your own money. Putting all payment processing into the hands of a few middlemen is a dangerous concentration of power.

It's sad that we make retailers eat a 3% gross tax on transactions in exchange for bullshit rewards. In the end merchants just bill the costs right back to us anyway. Individually we feel like we're doing well. We get flights or 1% cash back. In reality we're buying these things for ourselves and we're overpaying.

The payment processors can restrict your activity based on their assessment of risk. They can impose those restrictions ahead of any actual freeze.

Not different. But they can't freeze my cash.