When (not if) ISIS attacks the US, it will be determined the terrorists came across our Swiss Cheese border.
Why hassle with plane tickets and super tight security when you can just waltz across the border anytime.
And, of course, you can carry stuff across the border you’d never get in a plane.
I hear you and have said almost the same thing. However plane tickets are not expensive for an organization that acquires territory and loots. Why walk across the hot desert when you can fly in style? In fact why walk OR fly when you can walk AND fly? My guess is they have operatives coming in from all directions and all means of transportation.