I didn't even know Amazon was hosting WikiLeaks. AP says:

Amazon.com Inc. forced WikiLeaks to stop using the U.S. company's computers to distribute embarrassing State Department communications and other documents, WikiLeaks said Wednesday.

The ouster came after congressional staff questioned Amazon about its relationship with WikiLeaks, said Sen. Joe Lieberman, an independent from Connecticut.

On their Twitter feed, WikiLeaks posted a response to Amazon:

If Amazon are so uncomfortable with the first amendment, they should get out of the business of selling books.