Because the whole point of the phrase "miles per gallon" is that it implies the efficiency of using the gasoline. What you're talking about isn't getting 160 MPG. The car wouldn't be getting any 160 MPG -- because it wouldn't be burning gasoline in the first place to drive those miles. It would (most likely) be burning coal. And if you want a statistic on that, it should be a meaningful statistics, like the number or fraction of kilowatt hours at the electrical plug point the car will consume per mile.
I think you should probably give the early adopter customers more credit... for instance, gasoline was still fairly cheap in the US when the first Prius became a runaway hit. You're right, in the sense that if this car were judged solely on the number of miles it gets per gallon of fuel, that would make it look bad. But one should also be realistic, too. For the vast majority of Americans who don't own their own hydroelectric dam or windmill... some kind of fuel is going to be expended in putting electricity in the car's battery, and that fuel is going to have both cost and environmental impact associated with it.
