On the first viewing I actually found the acting laughably bad: for a world with no emotions way too many characters (Brandt, Dupont, Preston's kids) so obviously display them. Only on later viewings did I realize it was not a good, but foreshadowing: Preston is the only named character still on Prozium at the beginning of the movie.
The scene where Preston pistol whips a bunch of guys surrounding him isn't just to make him look badass: during training, gun kata is described as "treating the gun as a total weapon, each fluid position representing a maximum kill zone". This includes using the gun as a club, in case things come to that.
Gun kata relies on perfect emotional control. That is why DuPont engaged in Evil Gloating towards the end, and why they left Preston his guns; Take away Preston's sword, then take away his gun kata, and make him take the mooks on with nothing but his now useless guns.
Which adds another level of Brilliance: the result was pissing Preston off so much he went from barely controlling his emotions to full-on Tranquil Fury; in other words, he burned through all his crazed, unfamiliar emotions until he had only murderous rage: perfect, and completely controlled. Which made him even more effective at Gun Kata.
Related to the question of just how many people weren't actually medicating. All the people stiffly walking about, ignoring everything around them, etc. could in many cases just be people over-compensating for not being medicated. You don't need emotion to react to someone bodily running into you. Probably even the folks on medication are making a conscious act of acting like implacable stoics.
So Preston, for all his skill and dogmatic support for the system, turns out to be surprisingly bad at finding out who isn't on medications. But it all makes sense — the cabal that runs Libria probably aren't stupid enough to promote those who would expose their secret, eh? Preston, too inept in this regard to pose a danger, was deemed safe until he brought down the system for unrelated reasons.