This one is a Federal Grant Assurance that is being emphasized by the FAA. No cars, RVs, boats, or other non-aerial vehicles can be stored inside an airport fence without endangering an airport's Airport Improvement Program grant funds.
Most all those rules are driven by grant assurances, insurance, fire codes, environmental law, etc.
Does your airfield have a shop with an A&P that is paying rent, liability and other insurance? He's made an investment that can be harmed by someone working on someone else's plane from the trunk of their car. Another grant assurance requires a fair and level playing field which is interpreted as preventing freelance mechanics.
As a long-time aircraft owner and airport manager, I occasionally find it difficult and unreasonable to comply with the regulations I am supposed to be enforcing. (Now 36" shelves? I have no idea what's driving that one.)