Cough suppression is one of the first things that heroin was marketed for: http://i.kinja-img.com/gawker-media/image/upload/s--k12ZMisi...

You don't prescribe cough-suppressing opiates for a soar throat, the same way you don't prescribe a guillotine for headache.

Cough is a symptom, meaning there is a problem in the patient's respiratory system the doctor should be looking into. Suppressing it is the equivalent of pasting a post it with a happy face over the fuel gauge when the needle is approaching the Big-Red-E.