There seem to be a number of subsequent posts on here (this was from 21st March) similarly speculating that this ban might be for commercial rather than security motives.
There are recent news reports of a Trump meeting at the White House a few days ago, in which aviation risks were amongst subjects discussed with certain guests, and partly specific to (a) certain area(s) named at the meeting (but unnamed in press reports).
This would suggest that there are indeed security reasons.
Even in light of the recent stuff in the news, I'm not convinced that the move was purely for security concerns. The initial move was just a little too narrowly crafted so that it affected the ME 3 and not US carriers.