Surely this is letting terrorism win?

It all depends on what she knew or did not know. Apparently, he only got his passport back because he told them he was going on a pilgrimage to Saudi. If he just asked her to forgive him and gave no indication of where he was, where he was going or what he was going to do, there is not much that anyone could do in a short time - especially when several thousand miles away. *If* he told her he was going to blow himself up at 10:30 at the Manchester Arena then that is a completely different matter.