Because some people feel they "need" it. I guess I used the wrong word.
For example, I like to have a Ben and Jerry's Half Baked sometimes, and anyone that says I can't have it, that advice will just go in one ear and out the other. Sometimes we just have to find the balance in life that makes us happy.
To tell someone they can never have soda when they enjoy it, well I'd rather them have a diet than a regular.