Like people leave their engines idling while waiting for a pump and that’s not an issue… and if there was a chance of a spark traveling back along the line and ignigting the tank (and thus, the station), then why doesn’t this happen during normal operation?
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It’s nothing to do with a spark traveling in fuel lines or pumps or anything.
It has everything to do with the fact that a running engine is generating thousands of sparks per minute to keep itself running, and that fueling a vehicle emits a constant cloud of fuel vapors.
In ideal conditions the sparks and the fuel vapors never interact. (The spark stays in the engine, and the fuel vapors float away with the wind).
The problem arises if your ignition system had any damaged or worn out parts. A cracked ignition coil, a damaged distributor, a chafed spark plug wire, a cracked spark plug boot, or a broken spark plug could cause the spark to occur OUTSIDE the engine. And if you have a cloud of fuel vapors coming off of somebody’s filler nozzle it could ignite the vapors and cause a fire or explosion.
Most modern vehicles don’t have to worry about distributors, plug wires or old-fashioned coils allowing sparks outside the engine. But turning off the engine makes it so that there’s no spark to worry about until you close everything up and start the engine to drive away.
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