The handbook is fine. People just don't follow it. The fact that this boy's PFD came off just about proves that it wasn't worn correctly.
Yes, the boy was just being another teenage boy who thinks he is indestructible and hadn't clasped his pfd. But from the looks of it he might not have survived that soaking anyway. It sounds as if they hadn't intended on going through those rapids. The fact that they were so close is the fault of the Scoutmaster and his leaders, and whatever river guides they had with them.