Don't blame Smartpak because some people over supplement their horses. Lots of people are over medicated, but we don't blame Walmart!
The supplement industry is selling the idea that horse's can be healthy and live long comfortable lives without x, y, z and the kitchen sink. And these are products without evidence based science to back up the claims that they are making or to even prove that these products are safe. (Drug interactions have to be tested for, labelled for etc but that doesn't occur with supplements and mixing them may indeed lead to health issues.) Horse owners can go out and buy tons of supplements and dose them without having ANY advice as to proper use.
With medications people can make more educated decisions because the products have been tested repeatedly and must be marked with things such as the safe dose, reasons that should cause you NOT to use the product, possible side-effects, possible dangerous interactions, etc. And with you can consult a physician or even a pharmacist about the appropriateness of these products--people who have an education in pharmacology. Drugs are also not generally something that are advertised for you just to take on a daily basis for your whole life "just in case" you might have a problem later in life.