No it's not.
Can you explain how it isn't their choice? Mastercard, VISA and American Express turn down applicants ALL the time, so doesn't this go against what you are saying here?
I'm not saying it isn't their choice, but there is no real advantage to them angering politicians who could screw them on votes or funding at some point in the future.
Why would anyone do business with someone if there was even a tiny chance that they would get screwed over the next day to the tune of MILLIONS and MILLIONS of dollars. The fact that mastercard.com is down and their SecureCode service is offline has already cost thousands of small businesses millions of dollars. Do you think these companies are going to support something like Wikileaks after this event?
Anon has reduced Wikileaks' available resources by making people less likely to facilitate donations to them.
Visa and MasterCard can block payment providers if they believe those generate a lot of fraudulent activity, not because they disagree with those organisations activities.
It's Mastercard's choice whom they do business with, and by launching this attack Anon is only further solidifying the misguided popular belief that Wikileaks is akin to a terrorist organization. It may seem like a victory in the short term, but in terms of Wikileak's PR I think it's a huge setback.