I think every car insurance policy I've had since my first one (in 1977) has had this sort of clause - I thought they were universal. The argument is that aircraft are fragile and any damage is going to be expensive, so they don't inflict the cost of this liability on the >99% of their customers who never go near aircraft movement areas. I've worked on airfield sites since 1990 and my employer has always made it clear that 3rd-party damage cover whilst on our or our customer's airfields is the company's problem, not mine.
PDR