Over here (The Netherlands) the Hummer is also concidered a truck by law. It's not because some rich bastard wanted it but because of it's weight, the number of seats and the payload it can carry.
All those combined put it in the truck category instead of car. I'm guessing in the US this is also the reason, but you guyz might be right, Arnie could've had a hand in this.
Because it is a truck you have to get a special licence which isn't easy and cheap to get, thise licence puts you in the so called "profesional chauffeur" category.
You also get a discount on gasoline and purchase taxes, which is absurd in my opinion. What can you do about it? Making an exception in the law for a Hummer is undone, specialy since there are very few of them here in NL. On the downside however, because of the weight and the load it puts on the roads to road tax is much much higher then any other car.