While I agree with your statement, my state, Virginia makes a point of the responsibility of the driver.
The driver must ensure the passenger is seat-belted, as well as any persons under the age of 16 in the backseats buckled. And anything reckless on any of the passengers' parts is the drivers responsibility. Essentially they say that if a passenger acts out, stop the vehicle and demand cooperation.
My opinion would be that the driver deserves some reprimand, but NOT the charge of vehicular homicide. The reason I believe the driver deserves more is he was driving recklessly. However, he had no intention to murder any of his friends so why hit him with such a charge. I'm sure he already feels devastated that he killed a friend.