I'm not defending Brother Winston, who as far as I'm concerned is the most repugnant guy in this thread, but I disagree.
A nation can do whatever the heck it wants with their borders, whether leaving them open or closed. That's part of the point of government and a military. There has to be a zone of enforcement. Just like you have an invisible line around your property to which you say, "This is what I will use force to defend" so does a nation as an extension of an agreed upon policy.
Property, whether public or private, is determined solely by force and agreements based upon force.
As to your constitutional liberty for travel, it applies specifically to the movement between states, which is out of the domain of the federal government to interfere with.
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