Tyre Pressures, Fraser Island and Insurance cover

Up on Fraser last week and got talking to some friends who live at Orchid Beach about the perennial problem of Backpackers in rental troopies. I commented about how often you see them stuck in soft sand and he told me that when they pick up these vehicles they are given a list of conditions which includes "DO NOT LET THE TYRES DOWN".
Queensland Parks and Wildlife Service, who maintain the tracks on the island, used to have a sign at the entrance to the track from Middle Rocks to Waddy Point that said "Recommended tyre pressure 18psi". One day a troopie full of backpackers saw this sign and let their tyres down and shortly thereafter rolled the troopie, a not uncommon occurance. Went back to the rental company expecting to be covered by insurance but were told "No, your tyres were not inflated to the manufacturers recommended pressures as per the tyre placard and you are therefore not covered". Backpakers said we just did as the sign suggested. Insurance company said "Tough! Pay up!" Backpackers then attempted to get cover from QPWS who put the sign up but were told "No, it is only an advisory sign so no deal".
The washup is the Backpackers had to pay for the repairs to the Troopie, the sign has since been removed and Backpackers are no longer allowed to take their vehicles any further north than Indian Head.

The main issue this raises is what does "your" insurance company think of you lowering your tyre pressures and could they use this as yet another reason to avoid payment?

Cheers, John.