A couple of observations
The US drug official actually did make some veiled threats:
"I think the loss of the mutual cooperative partnership we've had with Canadians regarding our borders, regarding the integrity of the hemisphere, regarding our commerce, regarding the implications of trade and value to ourselves, the loss of that would be something truly to be regretted," said Murray, who repeatedly referred to the "unintended consequences" the new drug policies would bring.
He's not very subtly threatening trade, free access across the borders, and generally unspecified-but-sinister "unintended consequences".
As for an American going to Canada to smoke pot being immune from US law, don't be so sure. In some recent cases of Americans going overseas to have sex with minors, they were prosecuted under US laws that make it illegal to commit certain acts in other countries even if they're legal under the foreign laws. The US doesn't always make a clear distinction between domestic and foreign jurisdiction.
--Robert