The threat that is more serious -- is not the one to "him" and "his family" (i.e., in his office as President). It's the threat to the country as a whole. That's the serious threat. So, the actions taken were more *totally* for the country (and only secondarily, for him as a person).
Now, of course, they're going to protect him as a person -- but that's not the prime motivation. You've got to realize its the "Office of the President" that is at stake and what it would mean for the whole country if something happened to him. It would be a *big distraction* for managing and conducting the affairs of government (in the face of a terrorist attack), if the office of the President was "taken out" (although immediately it would fall to the second in line, obviously). It would be mighty distracting for all in the United States.