I haven't looked at the specific wording of the rules. An onboard fire system and a fire extinguisher effectively serve different purposes. An onboard system is something you pull when fire erupts to give you time to get out. You wouldn't realistically grab the extinguisher while you were sitting there and start spraying before you got out. Once you are out, you would like to have a way to spray whatever is specifically on fire. This is especially true if you pulling over caught the grass on fire or something. The onboard system isn't useful for this. One is to save you, one is to maybe save the car.

Richard P.