In response to an inquiry from members of Oregon's congressional delegation who were troubled by a notice on this subject from USPS officials in Portland, Thomas Marshall, the postal service's executive vice president and general counsel, reiterated that the Controlled Substances Act (CSA) prohibits advertisements soliciting the purchase or sale of Schedule I drugs. "These provisions express Congress's judgment that the mail should not be used as a means of transmitting advertisements for the sale of marijuana, even if that sale is allowed under state law," Marshall wrote in a December 15 letter. But he also noted that local postmasters have no authority to reject mail pieces for this reason.
"Per USPS policy based on the existing federal statute, local postal officials have been advised not to decide whether written, printed or graphic matter is—solely because of its content—non-mailable," a USPS spokesman told The Denver Post. Marshall said postmasters who come across marijuana ads can contact the U.S. Postal Inspection Service, and "the matter would then be turned over to the responsible law enforcement agencies for investigation if appropriate." Presumably that means the Drug Enforcement Administration (DEA), which under current Justice Department policy is supposed to leave state-licensed marijuana businesses alone unless their activities implicate "federal law enforcement priorities" such as interstate smuggling or sales to minors.
If that policy changes and the Justice Department decides to crack down on state-legal cannabusinesses, the fact that they advertise themselves probably will pale in comparison to the fact that they commit multiple federal felonies on a daily basis by growing or distributing marijuana. In that scenario, could publishers of newspapers and magazines be prosecuted for running marijuana ads? Not under the CSA provision cited by the postal service, which applies only to people who "place" marijuana ads. Hence this USPS policy has no practical significance that I can see.
Addendum: Marijuana Majority's Tom Angell notes that at least one U.S. attorney—Laura Duffy in San Diego—has interpreted the ban on placing ads for marijuana to cover newspaper publishers as well as radio and TV stations.