For the same reason we don't use other (still not torture) methods in domestic police interrogations.
There is a shoe scraping that every year calls the families of his victim just to torment them. I would be perfectly comfortable having him water boarded until he tells where he buried his victim.
Not so comfortable having the jerk that cut me off today water boarded.
Given the police attitudes of today, that's what I think will happen if we give the it's ok because it's not torture
approval to waterboarding — remember that the terrorists in this particular case are, under international law, unlawful combatants and therefore receiving much, much more in the way of rights/privileges than they should — I simply do not trust government to refrain from setting up precedent for violating the Constitution because they're terrorists
and then turning that justification upon the civilian population.
I find it quite illuminating to, upon any government action by "the elite", ask how does this increase government power?