Actually they are. The Border Patrol can set up roadblocks and do random searches within 150 miles of the border. In New England that includes places like I-91 south of White River Junction Vermont. They never really catch any significant numbers of illegal aliens there because Canadians rarely sneak into the USA, and there are an endless supply of back roads and highways between the border and the roadblock. But it is a great place to line up citizens and ask for documentation.
News outlets reported a few years ago that VT Sen. Leahy was stopped at one of the roadblocks and the Border Patrol agent, when asked by the Senator what the legal basis for the roadblock was, pointed to his pistol. Sen Leahy was reportedly not happy.
I'll be damned. They caught an honest to god communist, then let him go.