U.S. Customs and Border Protection officers at airports and land borders defer to federal law, rather than state law. This lesson was learned the hard way for a Chilean tourist who, after arriving at LAX to visit her US citizen boyfriend, was turned away by Customs and Border Patrol and permanently barred from reentering the US.
Her crime? On a separate trip earlier that year to Colorado, she had smoked recreational marijuana. Although Colorado had recently legalized marijuana, federal law still classifies marijuana as a Schedule 1 controlled substance. Any foreign national who violates federal law in the US is at a heightened risk for deportation and an indefinite ban from entering. However, as federal and state laws become increasingly discrepant from one another, it is easier than ever to fall into violation unknowingly.
Foreign-nationals must be extremely conscious of their actions under federal laws to avoid jeopardizing their current or future status in the US. When in doubt, it is best practice to err on the side of caution or consult with an immigration attorney.