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Because Visa and Mastercard are private companies that can decide whom they want or don't want as customers. They'll deny you from being a merchant if you have poor credit, too. They take on risk with every new merchant they accept, and I can see how WikiLeaks might pose a greater risk than most. If a group's supporters are willing to DoS visa.com and mastercard.com, I wouldn't put it past them to make large donations with stolen credit card numbers either, and Visa and Mastercard are on the hook for the chargebacks.

And, of course, since money is speech, it's fully within your rights to donate to wikileaks (cash, check, wire transfer, money order), just as its within Visa's rights not to accept Wikileaks as a customer.

EDIT: And just to be clear, I'm not trying to argue that there are no political motivations behind the decisions of Visa and Mastercard. I'm just answering the question about why this isn't for some reason illegal.