"Declaration of intent. The legislature finds that the abuse of controlled substances constitutes a serious problem for society. As a partial solution, these laws regulating controlled substances have been enacted with penalties. The legislature, recognizing a need for differentiation among those who would violate these laws makes this declaration of legislative intent."
Congress can pass a statute pretty easily if either a; rational, convincible, substantial or compelling reason exist. So, they choose for the safety, health, and to protect the morals of the public.
They can stop you from buying whatever they deem or even make you buy things: like insurance, brocolli, butt plugs, etc.