What's your proof they know? Clerks should stock whatever items sell well. National Enquirers and packs of Big Red gum are also right next to the register as well. Are those used for making meth too?
Well, they know now. Try to buy 5 boxes of certain cold medicines at a reputable store like Walgreens or Osco even in states without any laws restricting the sale- you won't be able to do it. All of these sleazy convenience stores had these medicines behind the counter long before any laws were passed- they knew the drug addicts would steal them- and not because they catch more colds. Ask one of these convenient store clerks for a Chore Boy pad and he'll pull it out from under the counter. And if it happens to be a store where you're a regular customer, take a look at the clerk's look of amazement when you ask for a product that only drug addicts purchase at his store.