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. :-/

[1] http://www.wired.com/threatlevel/2007/08/court-says-trav/