Because the actual mass of the caloric part of the food doesn't change when frozen.
If I have a chicken breast that is 200 calories, as long as I don't add anything to it, it's still 200 calories. Freezing and heating neither destroy nor add calories.
I wish they did. Destroy, that is.
"If you drop an egg, you don't say, 'Oh, shoot' and drop the other 11, do you?"
-Source unknown, but obviously brilliant
Reached goal 4/16/2010...but kind of afraid to look these days