Why? Those borders are completely arbitrary things we just made up
Those lands were conquered by the literal blood, sweat, tears and death of people, so they have a right to protect their territory. Besides letting anyone just walk into your country when you don't know their name, criminal background, diseases they might be carrying and their intentions for being there is extremely irresponsible and stupid as you can subject your own citizens to danger.
Those border are not arbitrary, that's just ignorance of history
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But in reality it does cause harm, and it's not illegal to be human only trespass.
Headline is the question
Do you mean across country borders or within a country?
I mean what was asked - all human movement
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