But that's not what is happening. Government subsidiaries (in this case, USPS) are selling lists of addresses to businesses based on one piece of knowledge: Zip codes. You provide tons of mail (literally) and a list of zip codes and USPS sees to it that the junk mail is delivered accordingly.

That's not a dumb pipe. ARIN/RIPE or Network Solutions aren't handing over lists of email addresses to spam.