These rules have a simple justification. If an attacker would like to breach airport security but is allowed to walk away once he has started the screening process, he simply needs to continue to try over and over again until he finds a vulnerable location. The Supreme Court has supported this idea [1] and I'm afraid that he may actually get legal action taken against him. :-/
As someone else in this thread has pointed out, that was a decision by 9th U.S. Circuit Court of Appeals, not the U.S. Supreme Court. From the article: "The U.S. Supreme Court has never squarely addressed the limits of the Fourth Amendment in the context of airport searches."
[1] http://www.wired.com/threatlevel/2007/08/court-says-trav/