That is a valid question though.

If dragons ate people and farted fire they'd be a flying crematorium

No. The mechanism that allows a dragon to "breath fire" is located in the esophagus. So they do not fart fire.

Source: The Internet

the most useful things in life are not learned in school

You know how you can breathe out warm/humid air to help clean eyeglasses? Dragons can do that but it's much hotter and it's fire.