You are photographed at road crossings. And I bet that there are enough security cameras in airports to get excellent pictures of everyone who looks like they've been to a strip club with Mohammed Atta and were flying on 9/11. One hopes these people have been dealt with on the QT and are being "interviewed" in some other country now.
You are photographed at road crossings. And I bet that there are enough security cameras in airports to get excellent pictures of everyone... The problem is that the sheer volume of images may never get reviewed until after the fact.
The value of doing it at arriving customs is that regardless of the volume (it's got to be a lot less than random security cameras) will ensure that it is feasible to review all images and isolate the "questionable" ones.
Seems to me that national security would make this an essential move.