[A CA Guy]

For most people, their common sense would tell them leaving a car running without the driver can get people hurt or killed. Thus you get this law.

#1: If car engages due to mechanical failure possible dead people.

#2: Some minor goes by, sees the operating car and takes their first self-driving lesson. Possible major problems right there.

Yes, a slightly small chance, but significant enough to not risk it.

[DouglasKC]

For most people, their common sense would tell them leaving a car running without the driver can get people hurt or killed. Thus you get this law.

If the CA is California then you likely don't understand that nearly everyone who doesn't have a garage in Michigan warms up their vehicle in Michigan in the winter. This guy got a ticket for something almost everyone has done or does...and nobody has ever heard of getting ticketed. That's why it's news. The cop is an idiot.