Infrared radiation. While thin sheet metal may not be able to stop a propagating flame for long, it will do a damn fine job of blocking radiating heat from melting the fuel lines...

Try taking a thinnest piece of sheet metal and putting it in front of your face while you stand next to a campfire and you'll see what I mean.
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I agree, but there are no exposed fuel lines, only the filler pipe for the tank. I agree the metal would help, but the rule allows for me to keep the plastic side panel in its place to shield the filler pipe. I suppose it has some value, but I wouldn't want to rely on it, just abide by it.