I think you might have misinterpreted that rule?
It is for light rail/trolleys (not buses) and only when you're on a two-way road and there's room to pass on the right. It also applies when they're moving, not just when they're stopped.
Basically, if a trolley/light rail has tracks in one of the left lanes of a two-way road, you must pass on the right unless directed otherwise by a traffic cop.
The reason is that these vehicles obstruct vision and you're not allowed to overtake and pass on the left when you can't see oncoming traffic or when approaching an intersection/grade/curve/oncoming traffic or your view of a bridge/viaduct/tunnel within 100 feet is obstructed.
https://www.dmv.ca.gov/portal/handbook/california-driver-han...
https://leginfo.legislature.ca.gov/faces/codes_displaySectio....