Cobb County police are arresting people who are stone cold sober for DUI by marijuana.
The goal of Drug Recognition Expert "training" isn't to weed out drivers under the influence of drugs. It is to provide a pseudo-scientific excuse to arrest people and convince juries to convict.
For example, a DRE-trained police officer will say that twitchy eyes indicates drug use, but will not say that a lack of twitchy eyes contraindicates drug use. Talking quickly is a clue, but so is talking slowly. Too much eye contact is a clue, but so is too little. Slurring words is a clue, but so is over-articulating your words. Fidgeting is a clue, but so is not moving enough. Are you too cooperative? That's a clue. Are you argumentative? That's a clue.
In other words, everything is a "clue" and nothing is a contrary clue. The officer begins with the presumption of guilt and uses his "training" to build a case.