Why Detroit?
DTW is only about 15 miles from Dearborn, which has a significant (30K) Arab population.
First possibility is that it is easier to blend in - compare Arabs traveling to visit relatives in Dearborn with Arabs traveling to visit relatives in... Round Rock, for example.
The second, and one that I find more worrisome, is that the residents of Dearborn are the intended audience. Either because they are being sent a message, or because of the opportunity to drive a wedge between the Arab community and their neighbors.
I presume that the employment numbers in Detroit still suck. So, lots of families in economic distress, a convenient target of Others to take the blame, and the rage of losing family/friends in a senseless act of violence? That's a clever combination, a cheap gamble to take for a significant payoff.