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There a lot of comments here on the behavior of lower level staff. They really aren't the problem.
Security hands lower level staff a set of constantly changing, often contradictory rules, often unreasonable rules. They have no power to question these rules. Then the security system trains these staff to confront passengers on the basis of these rules, and also to flag anyone who they think might be a problem based on behaviorial indicators.
Watch carefully the next time you go through the line--there is a definite hierarchy (and you don't see all of it). The frontline folks are pawns. They will be first and last to be fired if the security system steps over the line. The system will continue.