Why are the store clerks of the adult store surprised by Billy Pilgrim's purchases? (Chapter 9)

C. Billy Pilgrim is only interested in the science fiction novels

Another clerk came up to Billy and asked him if he was going to buy the book or not, and Billy said that he wanted to buy it, please. He had his back to a rack of paperback books about oral-genital contacts from ancient Egypt to the present and so on, and the clerk supposed Billy was reading one of these. So he was startled when he saw what Billy's book was. He said, 'Jesus Christ, where did you find this thing?' and so on, and he had to tell the other clerks about the pervert who wanted to buy the window dressing. The other clerks already knew about Billy. They had been watching him, too.