All NPC guards in supplements are pure Soldiers, unless I'm badly mistaken.
Thats a long way from saying "by RAW all guards are soldiers"
Luck isn't enough for them to see a sneaking thief. Natural 20 in a skill check is not automatic success, unlike in attack rolls and saves. So if the sneaking foes have stealth skills that are high enough, it is likely that they will never be spotted.
If the thief has high enough skills that a natural 20 doesn't spot them, then they damn well should get through.
Heh, but it just proves once more that you shouldn't trouble your head thinking about D20-based games from a simulationistic point of view, that they provided any hint at how the game world works.
It just proves once again that, when looking at the d20 systems in a simulationist way, you should actually bear in mind what they are trying to simulate. You seem to be complaining because a game designed to simulate Conan's style of high adventure doesn't accuratly simulate the real world.
well, there you go!