Actually, being from CA, I can chime in here as to the purpose and intent of this law...
Right now, if you are a teenager, it is illegal to drive with other teenagers in the car until you have had your license for a year (this doesn't make any sense to me whatsoever, but I don't make the laws, so...*shrug*
). So, kids were apparently getting around it by riding their friends around in their car trunks. I don't know much about it - all of my cousins except 2 are age 20 or older (the last 2 are 4 and 2 - we range from age 36 to age 2 - quite a spread, no?). I suppose my brother might know a bit more about this (he's a cop in our hometown). I might ask him, just for fun.
More new CA laws going into effect today are detailed here, which drew me because the headline was just so incredibly surreal:http://www.contracostatimes.com/mld/cctimes/16361336.htm
Enjoy our spate of new laws!