unless someone is underage in that pick up they are legal
Not in California, they're not - 18 and over still can't ride in the back.
That was thanks to a family of undocumented migrant agricultural workers crammed into both the cab and box of a pickup, the driver of which decided it was ok to turn right at a red light as a big rig came through the intersection.
It happened in North Highlands, CA - right at the intersection of Watt Avenue and Gate #2 of McClellan AFB. My billeting was about 25 yards from that intersection.
It took a lot of fire trucks and hoses to wash that asphalt off.
California passed a "no passengers in pickup boxes" law very shortly thereafter.
To date, there are about 28 states with such laws on the books.