I’ve never seen one of those signs, but I drive a pickup and could suggest the issue is a sidewalk. Backing a truck into a spot can cause the bed of the truck to overhang whatever you’ve backed into, which could block a sidewalk or other walking path. Typically there is less overhang from the front of a vehicle to the front wheels than there is from the rear of the vehicle to the rear wheels, so pulling into a space forward wouldn’t impede any walking path as much.
I’ve never seen one of those signs, but I drive a pickup and could suggest the issue is a sidewalk. Backing a truck into a spot can cause the bed of the truck to overhang whatever you’ve backed into, which could block a sidewalk or other walking path. Typically there is less overhang from the front of a vehicle to the front wheels than there is from the rear of the vehicle to the rear wheels, so pulling into a space forward wouldn’t impede any walking path as much.