Why shouldn't junk food be banned?

I'm not saying I'm for or against banning junk food, but if you want to ban you have to give answers to three questions:-

1) What is the precise, legal definition of junk food?

2) Why can't people be allowed to enjoy a little junk food occasionally?

3) Why do you get to decide what somebody else does or does not put in their mouth?

This is even before we get onto matters of social conditions and the cost of junk food versus non-junk food.