If anyone can mine a thing for themselves (i.e. create more supply of it), then that thing is a commodity. (Gold, for example, is a commodity.) By definition, no one entity can really restrict the supply or trade of a commodity; and thus the government can't really make useful laws about what any one entity does with their holdings of a commodity.

Any valuable thing that isn't a commodity, is a security. Every fiat currency is a security. Stocks and bonds are securities. Etc. The government can regulate securities (or rather, regulate their issuers, because there is a clear issuer), and so they do.

The definition of security isn't really vague; it's just a definition of exclusion. Replace security with "non-commidty" whenever it comes up and things might be clearer.