Very first sentence of AWS Service Terms:

1. Universal Service Terms (Applicable to All Services) 1.1. You may only use the Services to store, retrieve, query, serve, and execute Your Content that is owned, licensed or lawfully obtained by you.

http://aws.amazon.com/serviceterms/

I'm not a fan, but they made those terms for a reason. This is simply Amazon enforcing the rules that they set in place a long time ago. Wikileaks should have read the terms before they signed up.