My initial thought was the same, but then I wondered: if this computing is done inside a home, isn't it the same as just heating the home? It would only be wasteful if the home is actually being cooled by air conditioning.
While not as mathematically elegant a comparison, you could certainly run the numbers for gas or any other heating method and find a similar cost comparision. They key point is that a computer heats the way a space heater heats, and heating your home with space heaters is expensive even in spite of the 100% efficiency at turning electricity into heat.