I've seen it arise with visitors to the United States. There was a case that got some publicity recently in Colorado of a woman who was travelling to the United States to meet her boyfriend, who lived here. She lived in South America, and she was denied entry because she admitted that she had used marijuana once on a previous trip to Colorado. Her answer was, “Well, it's legal in Colorado.” The patient people at the border explained to her that under state law maybe, but in the United States it's not. Because she made that admission, she is no longer eligible to travel to the United States.
On September 12th, 2017. See this statement in context.