Uhmmm, the hose on the tank could spring a leak in the tunnel and blow the whole thing up

. I'll bet they make you turn off the LP gas valve when you're on a ferry boat. And the flame on the fridge is also, as another poster pointed out, not exactly open, like a candle or a blow torch, in addition to not having a pilot light. I say NO FLAMING TORCHES IN TUNNELS, unless you're welding on the boring machine.
Seriously, though, LP in a tank is under pressure and if it does spring a leak it creates a significantly greater danger of explosion than if a gasoline line or the gas tank starts a slow drip. I think that's why they want them turned off in ferryboats. If you've ever been around a boat fire, it's pretty sobering. No place to go but into Neptune's lair.