I think the reason is due to spills. Sometimes the mechanism does not cut off when the tank is full. Gas then overflows the tank and onto the ground.Sorry this is later response ...but I have an idea that gas stations can require customers to hold the pump handle while pumping gas as maybe a deterrent to drive offs?
If the customer went into the store while trusting the mechanism, the amount of spill can be pretty significant. I thinks some states have banned the mechanism as an environmental hazard.