Freedom of information. Just because it's out there doesn't mean it's going to happen. It's like the age old racist and antigay propaganda, reading about or associating with these people does not make you one of them. Likewise (as minimus pointed out) reading this material does not equal acting on it. The issue is with the person that acts on it, not those that have something to say on the matter. We all have dark thoughts on occasion, the difference between most people and Tim McVeigh's of the world is acting on it. Sociopaths, murderers etc will do what they do with or without someone publishing a book.
If we're going to ban a book (or books) based on damage caused or lives lost, it should be this one: