Why were John and Lorraine arrested in Pigman?
John and Lorraine are not actually arrested, because Mr. Pignati declines to press charges. When the police arrive at the party, however, everyone else had run away except for Lorraine and John, and after talking to Mr. Pignati, the officers take the two remaining teenagers home. John is passed out from drinking so much, and Lorraine had been caught at the house because she had stayed behind trying to get him up off the floor. One of the policemen helps Lorraine get the insensible John into the patrol car, "roller skates and all".
In the patrol car, the policemen tell Lorraine that they are lucky Mr. Pignati did not want to press charges. The last they had seen of him, he had gone upstairs crying, his wife's ripped dress over his arm. The policemen wonder what kind of parents Lorraine and John have to let them run wild like this; Lorraine says her mother will beat her when she gets home, but the policemen are unsympathetic. Lorraine's mother does indeed slap her around when she gets home, but after an emotional scene, seems only to be concerned that Mr. Pignati might have tried something "sexually" with her daughter. Lorraine tiredly accepts the fact that her mother will never understand. John's parents react with much more reserve. His father decides that John needs to see a psychiatrist.
Neither Lorraine's nor John's parents are concerned about Mr. Pignati. Worse yet, neither really cares to listen to what their children have to say (Chapter 14).
John and Lorraine were not arrested. They had a party which got a little out of control. John had a little too much to drink and the party became too large via word of mouth. Mr. Pignati arrived home from the hospital after his heart attack and the police escorted John and Lorraine home.