Though everyone knows that certain kinds of speech, such as pornography, are against the law, most of us don't think about the web of social, nongovernmental constraints on legal but disfavored speech. Unpopular ideas are marginalized in our society, restricted to the fringes of public discourse even without the need for any governmental action. Many rely on advertising, or own other businesses that do, or are simply owned or controlled by people whose personal involvement in the social web of contacts and constraints guarantees moderation in ideas. No idea sees the light of day until it has been turned over, examined from every angle, and pronounced fit for human consumption. Editors approve articles and books, and are managed by publishers who sometimes intervene in content.
Committees decide what news stories to cover and which to ignore.
Our courts have confirmed that the First Amendment to the US Constitution protects many works we don't really want anyone to see. The Anarchists' Cookbook is an example. A morally repugnant compilation of bomb and murder recipes, it has been selling steadily for 25 years. You have probably never seen it in a bookstore or a library. The government cannot stop it from being distributed but, where the government leaves off, the web of social controls takes over.