This is no great surprise. It's pretty obvious that Wikileaks is not complying with US law -- whatever you might think about it -- as publishing classified documents online is a crime. Hence any US company knowingly helping someone perform illegal acts it itself committing a crime, under US law.
If the company doesn't do any business in the US, no problem, but Visa and Mastercard do...
(Note I'm not giving any opinions one way or the other here, but just stating that compliance with the law is what companies that don't want to get massive fines do.)