Nor do I, but I also don't think we should restrict everyone just because of some stupid people. As the saying goes:

If you make something idiot-proof, someone will just make a better idiot.

Put up sufficient warnings, perhaps warn them verbally before purchase, and then let people do what they want, provided it won't hurt anyone but themselves. Obviously there should be exceptions for when someone is impaired (e.g. a bar), but if they seem to be of sound mind, let adults make their own stupid decisions.

I don't expect a cashier to know that someone could die when purchasing their product, nor do I think the business is obligated to do anything more than warn when it comes to relatively harmless products like caffeinated beverages.