"I do not think it is that clear-cut, even for car rentals"

Well, it is that clear cut. As the article notes there is a law which unambiguously states that car rental agencies are not liable in this scenario: http://gregjohnson89.wordpress.com/2010/10/28/vicarious-liab...

As the article further notes, the only question is whether a loophole in the law's definition may cause Ms. Fong to not qualify as a car rental entity.