He's not the Messiah, he's a very naughty boy
"The leaks occurred solely because of the failure of American authorities to properly secure the data." No, the leaks occurred because of crap security AND because a trusted individual who had agreed not to do so stole some data. IANAL, but it sure sounds like a US crime has been committed by a US citizen.
This individual chose to pass this data on to Wikileaks, who knew perfectly well that it had been obtained unlawfully. I'm pretty sure that selling property known to be stolen (albeit in another country) would still be a crime in the UK and elsewhere. I'm not sure of the position if you give it away, but don't forget that Wikileaks are doing this for publicity and to attract donations.
Irrespective of this, Wikileaks are making the information available to US citizens against the wishes of its owner, so they are (prima facie) committing a crime in the US. They may wish to argue that this is either in the public good or that the US public is the true owner of the data, but they're free to do so in the US courts (good luck with that).
So it's pretty clear that if Mastercard (a US company) processes payments in the US on behalf of Wikileaks, it may well be breaking the law. In those circumstances, what would you do?