You use the word "prohibition" and are assuming that part of this "prohibition" is to appease the DEA (which is probably true), but another part of it is to protect Costco from litigation for the wrongful death of someone who ODs ingesting both imodium and narcotic drugs, or other detrimental health impacts from the overuse of this product (including fecal disimpactions on those not savvy enough to use stool softeners). We don't know that such litigation has not already happened. Whatever the federal regulation is, is what it is, and Costco must by law abide with it, but if industry standards are changing and overdose patterns are being connected to the Imodium product, both the manufacturer and the retailer have to take steps to prevent themselves from litigation if they are putting a potentially defective product on the market. Imodium taken in normal quantities is probably not a defective product, but for purposes of statutes and regulations, if sold in a sufficient quantity to enable the causation of overdose in combination with other drugs, it may be deemed a defective product in the market place. The resulting lawsuits would burden Costco, if they did not protect themselves on this one. One thing the public does not understand is that regardless of merits, lawsuits have to be defended and they cost money to defend. I am an insurance defense attorney, have defended many frivolous lawsuits in my day and I got paid by the hour on every one of them from start to finish. You do not wave a magic wand and make a frivolous lawsuit go away. But even if it has some slight merit to it, it will need to be defended in court and at great cost. Multiple that by 10 or 100 or 1000 and it gets worse. Stores like Costco are huge targets for lawsuits. I know because I have defended restaurants and large corporate retailers, and with large corporate retailers and wholesalers, they are huge targets. In my experience the only less likeable defendants in front of a Connecticut jury are tractor trailer drivers, due to the nature of our highway system and I-95 being a corridor through which there is a very heavy tractor trailer traffic and many serious car accidents caused by this traffic. Juries do not like some of the large corporate retailers operating physical stores where people slip and fall and have accidents, and they like them even less in this age of e-commerce and Amazon.
I am also questioning your use of the term "majority" and "minority". Is the number of potentially affected narcotics addicts a "minority" in comparison to the people who need to buy more than one bottle of imodium in a year or two? I think not. Certainly not from the DEA's, or any governmental regulatory agency standpoint. I use Imodium as well, although for some reason I do not need very much. My body reacts very sensitively to it, meaning 2 pills will REALLY thicken me up and sometimes I will only need one in a day. I seem to be lucky on that count.