The book is not available in every country, nor is it accessible to everyone.
And frankly, prevent sales from what? It's already a bestseller. People who wants to buy it already did so.
I wouldn't have bought it, but since it's there for free I'm going to read it. Your argument is similar to the fallacies where people think that illegally downloading games somehow hurt sales. It does not, it's free marketing. If anything, this gave a lot of visibility to the book.
Your hatred of Wikileaks blurs your vision. The book is released uncommented, unedited. It's the real deal.