When discussing his new book “The Enemy of the People: A Dangerous Time to Tell the Truth in America,” at Point Park University’s Pittsburgh Playhouse on Thursday night, Acosta faced cries of “InfoWars,” “fake news” and “hateful” from members of the audience.

As the clock in Market Square reached 12:30 p.m. Monday afternoon, about 115 young adults dressed in jeans and white T-shirts began flowing in around the perimeter of the square, moving their arms and...