Theres always a chance of a backfire and/or a leaky exhaust system igniting fumes. Combine that with some goofy fuel filler locations (remember the ones down low behind the license plate on GM cars?), hence the law. Even if it were legal, its wasting gas.

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1985 380SE Blue/Blue - 230,000 miles
2012 Subaru Forester 5-speed
2005 Toyota Sienna
2004 Chrysler Sebring convertible
1999 Toyota Tacoma