Comment: An answer for 'why'.

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An answer for 'why'.

A clue to the answer for your first question was reported by the BBC.

From the viewpoint of a UN Migration Minister: "The EU should "do its best to undermine" the "homogeneity" of its member states, the UN's special representative for migration has said." It was for the good of the EU economy of course "however difficult it may be to explain this to the citizens of those states".

He wasn't speaking about the US but maybe this is a partial explanation for why our own borders have been allowed to be so porous. Economically, it is not as good for US or EU workers as it is for the 1% who can manage a cheaper labor market.
http://www.bbc.co.uk/news/uk-politics-18519395