Which is a US centric approach.
And even within the US it won't do much good.
Spammers will just set up PO box corporations in locations where the products/services they're selling aren't illegal or there where they can not effectively be touched because there are no extradition treaties.
As to prescription drugs offered through spam, that's a good example of where this is already happening.
The resellers are in Canada (at least for the law) and other places where the sale of those drugs is legal.
Yet I'm in the Netherlands where it's illegal to sell any prescription drugs except when you're a licensed and registered pharmacologist (even hospitals need them to sign off on prescriptions).
Under our law those spammers could be charged with offering illegal drugs, but under the law of the country they're shipping from they're doing nothing wrong.
And even if they're illegal in that country as well the cost of legal proceedings would be such as to make it impractical if not impossible to go after them.