The law is very strict about carrying alcohol or drugs in a vehicle whether the vehicle is on or off the highway. You must not drink any alcoholic drink in any vehicle.
A container of liquor, beer, or wine carried inside the vehicle must be full, sealed, and unopened. Otherwise, it must be put in the trunk or a place where passengers don’t sit. Keeping an opened container of an alcoholic drink in the glove compartment is specifically against the law.
In a bus, taxi, camper, or motor home, the law does not apply to non-driving passengers.
So it looks like the non-drivers can booze it up in the back in Cali, as long as the driver isn't drunk. I'll be looking into the other states too, and post them as soon as I can find them.