By its nature, a gas station can have areas where flamible fumes can build up from such sources as spills and leaky filling hoses and nozzels. A parked, running car stands more of a chance to produce enough heat to ignite such gas pockets. Sourced include hot exhast manifolds and backfires.
Your car may be in great shape, but it's not a particularly burdensome saftey reg to require turning off all cars while filling up, just to reduce the risk posed by someone's faulty car. Many localities also forbid cell phone use near a gas pump because there have been reports of ignitions caused by radio interaction with the electronics in modern gas pumps. Again, it's not likely, but it's too easy to eliminate the potential hazzard.