Black's Law Dictionary (6th ed.) defines duress as "any unlawful threat or coercion used... to induce another to act [or not act] in a manner [they] otherwise would not [or would]". It also defines "coercion" as "force or threat of force." Therefore, in court, the passengers going through the scanners are consenting. We've already cited cases on point, see Hartwell above.
Economic necessity might be enough to show a contract of adhesion, but not lack of consent for fourth amendment search and seizure purposes.