Well, the truth is, they aren't. According to an article on the Weight Loss International site, there is only one negative calorie food: cardboard. Since our bodies lack the enzymes needed to properly digest cardboard, it would pass through our body without being broken down. This passing process would take a very minimal amount of energy.
The foods on the "negative calorie" list include some really healthy choices though, like celery, broccoli and lettuce. So even if these foods don't help you lose weight by burning extra calories, they will help by being better for you than donuts and cupcakes.