Why are Negative destructive actions so bad?
When you apply, for example, white balance to an image in-camera, you do it to the exclusion of other white balances: Once it is applied, all the data that wasn't used to apply that particular white balance is discarded before it is ever written to a file. All the 'destructive' actions are like that, discarding image data every step of the way. Once a destructive action is completed, you cannot undo the actions.
If you do try to correct them (say, your camera wrote an image with a color cast to the memory card), you will incur even further data loss when you attempt to fix the problem.
Think of 'negative destructive' actions as one-way streets: Once you've gone down it, there's no going back.
How is editing in RAW different?
When you are editing RAW files, you never 'lose' the original data, and it is possible to change your white balance, sharpness, saturation, etc after the fact. If you change your mind, the original data is still there, and you can do so without degrading your image.
To coninue the analogy of above: Non-destructive editing on RAW files is more like a two-way street. If you get half-way down the road and discover that it takes you in the wrong direction, you can spin your car around, go back, and try another path.
If you're still not quite convinced, this article on froknowsphoto.com has some excellent side-by-side comparisons of post production flexibility on editing JPEG vs RAW files.
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