Cough medicine isn't the ingredient used in meth production. Pseudoephedrine as an ingredient in pill form is the culprit in meth production. Pseudoephedrine is a decongestant and isn't a cough suppressant.
Oklahoma was the first state to enact a law where medication in pill (or dry) form containing pseudoephedrine had to be placed behind the pharmacy counter. Liquid cold medication which contains pseudoephedrine is still sold off the shelf here. I can still go into the local WalMart or pharmacy and find Nyquil (on the shelf) which has pseudoephedrine as one of the ingredients. From what I've been told by our local Undersheriff, only the dry form can be 'cooked' to form one of the ingredients for meth.
What you say makes sense, I guess in liquid form it is diluted and dispersed throughout the liquid. I'm sorry for any inaccuracies, I never touch that stuff. I even have a cold right now and won't touch it. I stick to Vick's!