While I understand the logic of your point, I think the assumption you make is that MasterCard will add in the costs of a possible DDos attack when considering whether they made the right business decision.

And I think the answer is no, although it's arguable. There is surely a point at which MC will try to do the "right" thing instead of the profitable thing, and it's easy to see how that point is between cutting off Wikileaks (which arguably is harming US national interests) and changing business strategy to appease hackers.