Dontzap is the app that prevents ctrl-alt-backspace. This would mean that it is
installed by default, but for some reason it was not installed on yours. Perhaps
you ran an earlier version that did not have it and for some reason it was never
installed. It does not really matter in any case as you seem to know what you
are doing and can either install it or not as you choose.

There is some discussion that dontzap does not work on some computers. See:
http://ubuntuforums.org/showthread.php?t=249899 and
https://wiki.ubuntu.com/X/Config/DontZap

I am sure that we have not heard the end of this discussion. I am a big defender
of Ubuntu/ Canonical because I respect what they do, but this is wrong headed
IMO. I have never heard of anyone simultaneously pressing three keys that
require two hands by mistake. It takes caution to an extreme bordering on
paranoia. Is this signs of more to come to protect users from their own
stupidity or just an aberration? I hope that it is just the latter.