This does not occur in a legal vacuum; for example Hit Man: A technical manual for independent contractors is an incredibly detailed description on how to commit a murder, hide the body etc, and was found to be used as a manual in a triple murder. It was found to not be protected under the first amendment (although it never reached the supreme court). There was a genuine concern that this book carried a risk of consequences and had already inspired a multiple murder.
It raised clear issues which were never settled conclusively.
Censorship is easily abused, but it would be a strange person who would campaign for free speech without limit. The argument that is usually made is that of a slippery slope; we've been going down that slope for hundreds of years and still have no Ministry of Love or thoughtcrime.