I don't see the distinction as absolute (or even apparent). Iraq is the most recent and glaring example of a terror campaign with non-defensive political goals, started by a nominally democratic (we can quibble about definitions of democracy, but let's not) state without the consent of most of its people. On the opposite end, one might argue that the actions of a terrorist group, while violent or repugnant, might be committed for the defense of their people against some oppressor. (Just because the suicide bomber herself dies doesn't negate dialogue with surviving members of the group any more than the death of an individual soldier negates dialogue with a state.)

On a more abstract level, it strikes me as the difference between two murderers. One has a knife, and stabs his victim in a heated, passionate moment. The other has pause to consider it, and hires an assassin to shoot the victim. All other things equal, both acts are reprehensible, but the law might assign manslaughter to the first, and first-degree murder to the second.