... It's illegal in this country to distribute this information. Amazon had a legal obligation to terminate their relationship with Wikileaks. ... It is most certainly illegal. Perhaps you should go read up on Civil Disobedience.
Please remember that disclosing classified material is only illegal if the person has a security clearance. Once the material is made public (i.e. leaked), possession of it by a non-cleared person is not a statutory violation unless, of course, that person used illegal means to obtain it (e.g. breaking and entering).
(You may have trouble with the Special Rendition team, though...)
Conceivably, though, possession AND DISCLOSURE of, say, TS/SCI information could rise to the level of treason, particularly if the data was sold.
While Amazon may have a contractual obligation to do what they did, I don't believe they were legally required to do so.
Perhaps you should read Executive Order 13526 (and 13292, perhaps). Issued in 2009 and 2003, respectively, they codify (alongside 32 C.F.R. 2001), the United States' legal framework for protecting classified data. All of them are quite a bit newer than Thoreau's writing.