Why, if you have an exposed filler pipe for the fuel, do you have to cover it?[/b]
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.