When Ross Ulbricht, the alleged operator of a massive online drugs black market, was arrested on October 1, 2013, FBI agents immediately snatched his open laptop away from him. Just moments earlier, he had been sitting down in the science fiction section of the Glen Park branch of the San Francisco Public Library, chatting with “cirrus” about a Silk Road forum post that needed to be moderated. Then a fight erupted behind him—a man and a woman, seemingly a couple, shouting and screaming...