Kathleen Sweet's book, Terrorism and Airport Security (2002) discusses the legal precedents for consent to be screened, particularly the notion of "implied consent."

Sweet doesn't specifically address reverse screening, so perhaps the legal justification should be reviewed. The question, of course, is "When does a flight end?" This would include my pet peeve: post-arrival screening of international passengers so that they can make it through the secured area to the baggage claim. This occurs in Atlanta, Cincinnati, Cleveland, Seattle, and some others.

I agree, it should be clearer to passengers that they can be searched as many times as the authorities see fit.