I would guess that they use those "Multi-Purpose Dry Chemical", aka "A-B-C" (mono ammonium phosphate) extinguishers because they're cheap! Otherwise, I agree that they are very corrosive! "Regular dry chemical" aka "B-C" (sodium bicarbonate) costs more, but is non-toxic, non-conductive, and non-corrosive. I don't think we have too many cars running that have much wood in them, so we shouldn't need the "A" component!
I submitted the rule change requiring push trucks to have a (minimum 10) "B-C" rated (rather than the obsolete rating in pounds) extinguisher a few years ago.
I work as an aircraft inspector for USDOI and that's what we're requiring on helicopter fuel trucks. I believe that the FAA prohibits mono ammonium phosphate extinguishers on airport ramps, too.
Please note that this applies just to "portable" fire extinguishers, and not the fire extinguishing systems which are required in the race cars. That's a whole different ball game, wherein system ratings in pounds still applies, as different extinguishing agents are utilized.
For more info, see the National Fire Protection Association's NFPA-10 Manual on Portable Fire Extinguishers. The bad news is they charge for a copy, but there may be some out there someplace on the internet.
Jeff in Boise