That's not a drill.
Passengers require no training to learn how to stand still, and thus do not require practice.
A drill is an exercise for a complex and critical task. In the hospital, we sometimes have code drills, as a chance to ensure that everyone knows the steps to follow in critical events. It does take some practice, proficiency, and comfort to get all of the lines in the right place, the monitors on, the drugs pushed, etc. In a best-case scenario, the drill ensures that the choreography works.
A drill also means that those involved in the drill would be present should the critical event occur. In this case, participants in the drill were travelers, not necessarily from Atlanta, and only at the airport for a brief (but ugly) moment in their lives.
Yelling "freeze" is a power trip. Regardless of who came up with the idea, this is merely a chance to inconvenience, annoy, and humiliate the traveling public, while giving uniformed "officers" a trial run at their inflated authority.
PLEASE, write down as many details as possible. Tell the TSA, the FSD, and the Atlanta Journal Constitution. You might also wish to tell the Atlanta Airport Police that emergency drills were taking place at their facility--were they aware of this? I think it's always worth asking the question, "What was the goal of the drill?"