Makes sense to me. Wikileaks is violating laws by posting other people's private and classified information. Amazon has no business aiding them in criminal activities. Free speech != exposing classified documents. Free speech is about original content. You can say / publish whatever you want online. But you can't take other people's content and post it, especially when it could endanger lives.
The Supreme Court has ruled that publishing classified material is, at least in certain circumstances, protected by the First Amendment's guarantee of freedom of speech and of the press. New York Times Co. v. United States, 403 U.S. 713 (1971). The First Amendment's protection isn't absolute, of course; publishing the details of imminent planned troop movements most likely would be punishable if such publication would place our troops in severe danger. Given the sparse caselaw on the subject, it is an open question whether Wikileaks' publication will be considered protected by the First Amendment under the jurisprudence of the Supreme Court, but I would guess that Wikileaks' actions are indeed Constiutionally protected.