Just wait for the howls of outrage if Canada ever legalizes marijuana and the US denies entry to those who have partaken.

See INA § 212(a)(2)(A)(i)(II) and 8 U.S. Code § 1182

Under INA §212(a)(2)(A)(i)(II), aliens are inadmissible to the USA who admit to having committed, or who admit to committing acts which constitute the essential elements of a  violation or conspiracy to violate any law or regulation of the United States relating to a controlled substance as defined in   21 USC § 802 (includes marijuana).  And there is no 30 grams or less dispensation under this section.