Well how do you tell your kid "no" when he asks for candy, when he can get as much as he wants at friend's houses, school, the library, or basically anywhere outside your house?

Edit: better exaple would be cigarettes, since that's something we as a society recognize is bad for kids and generally require proof of age if there is any doubt. Imagine if all your kid's friends smoked, and there were cigarette vending machines at school, and all you could do was say "no."