The passenger asserts at booking that "I will be at the boarding gate before the plane leaves because I intend to occupy seat X on your aircraft" but that's a lie, because they have no such intention to board the aircraft. That's the fraud, and that's why it's low-grade, because people miss flights accidentally for numerous reasons, this is just a deliberate missing of a flight, and taking away resources from someone else who should have them. Although overbooking, bumping, and standby are all policies in place to deal with the inevitable missed flights, whatever their reason.