Why? They are legal adults who can take responsibility for their actions. They've been irresponsible and now are facing the consequences.
Because the legal consequences for something so minor are massive in comparison in the United States. While they are in Cali and the sentencing will be minimal and both will have a very enjoyable probation period, the whole event of the police asking you questions, detaining, searching, and then arresting you is hands down the worst experience I have ever had, I can only imagine it was the same for them.
You know that feeling on a roller coaster when your stomach drops? Add that to the feeling of all hope lost and you got yourself a charge, who knows what they got.