Marijuana may be legal in some states in the United States, but it is federally prohibited. This means it is illegal to bring marijuana bought in Canada across the border into the United States.
This is the case even if you are flying into a state where it is legal, such as Colorado. Crossing international borders with cannabis is illegal.
Flying with pot from Vancouver to Montreal, on the other hand, is perfectly legal so long as you meet the age requirement and carry no more than 30 grams of cannabis.