Since I don't like to call in the show, Butter Spray does have calories, just they round it down with the suggested serving size.
"If something seems too good to be true, it often is. "I Can't Believe It's Not Butter" spray has ZERO calories if you use just a few spritzes. However, if you do what most of us do and spray like there's no tomorrow, you can rack up calories and fat grams pretty quickly. Here's the scoop: 12.5 sprays is 10 calories and 1 gram of fat, 25 sprays (1 teaspoon) is 20 calories and 2 grams of fat, and 37.5 sprays is 30 calories and 3 grams of fat. In case you're curious, the entire bottle contains 900 calories and 90 grams of fat! YIKES! Obviously, when it comes to this stuff, stick with spraying as opposed to pouring."