[jimworcs]

Airlines can only rebook passengers on airlines with whom they have reciprocal ticketing agreements

If the airline cancels, the law should mandate that a customer can purchase an alternative routing with another carrier if the original carrier is unable to re-route to land within 6 hours of the originally scheduled landing time. In this case, the passenger could then have booked on Virgin Australia and billed United. That would put a stop to this nonsense of customer service agents "not trying very hard" to re-route passengers, whcih happens all the time.