A higher rate of speed does not always mean a lower fuel mileage rating. It depends on the specific speed and vehicle. So just because someone is driving faster than you doesn't mean they are wasting fuel (or that they are idiots).
Driving slower in fact does produce better gas mileage. When a crew chief in NASCAR wants his driver to save some gas, how do you think that driver goes about it when he is leading a race? And have you seen how some speed and weave in and out of traffic? They would indeed be idiots when they are putting others in danger.