Without any border controls people would be forced to assume that anyone from outside their country was a thief or con-man looking to find some quick victims and head home.
That wouldn't be pleasant for either side.
That wouldn't be pleasant for either side.
In a case of genuine open borders criminals can leave the state quickly to avoid punishment. Then move on to a new target country.
This doesn't happen only because no country signs an open border agreement without also setting up a legal framework to punish criminals who would try to exploit borders to escape punishment.
The reality is that there are bad actors out there and if you aren't stopping them at borders then you'll have to start watching out for them closer to home.
High trust nations are like unpatched machines. You need a firewall.
Schengen has a shared outer border.
Presumably, if one demonstrates mastery of language(1) and cultural knowledge(2), he can be assumed to be a countryman.
1) This practice dates back to biblical times. "Shibboleth": mid 17th cent.: from Hebrew šibbōleṯ ‘ear of corn,’ used as a test of nationality by its difficult pronunciation (Judg. 12:6).
2) Common example: During WW2, if a suspected spy were encountered, mastery of language and accent may not be enough to assess his status. Cultural questions were asked: Where does the NY Mets play? What's Mickey Mouse's girlfriend's name? Questions could also be about celebrities, movie stars, music, novels, etc.