Actually, if you're going to stick with cars, it's like a safety and emissions check in realtime. If your car is spewing excessive pollution or presents a hazard to other drivers (critical safety features like turn signals, head lights, tire treads, etc., missing or malfunctioning) they don't let you go around being a hazard on public roads. It makes sense for ISPs, in a uniquely capable position to detect it, to disconnect systems that are spewing malware and presenting a hazard to other computers on the network.