One should be very careful in ascribing rational motivations to things the Trump administration does - even his rhetoric (vague allusions to the attacks of September 11, 2001) isn't borne out by the executive order. He did not ban immigrants from a single one of the countries of origin of the 19 attackers.
http://www.pewresearch.org/fact-tank/2017/01/27/key-facts-ab...
One should be very careful in ascribing rational motivations to things the Trump administration does - even his rhetoric (vague allusions to the attacks of September 11, 2001) isn't borne out by the executive order. He did not ban immigrants from a single one of the countries of origin of the 19 attackers.