Originally Posted by exerda
I wonder what the jurisprudence behind that characterization is? One would think that it would likewise allow them to snoop in on and record your conversations in line and at the checkpoints, or elsewhere in public, since these aren't "searches," and use those as evidence against you in court, which in general can't be done.
This is something of a gray area. Many Fourth Amendment opinions use an objective "reasonable expectation of privacy" as a standard for the admissibility of some evidence, especially that gathered without the consent and knowledge of the target. If you are in a public place, your reasonable expectation of privacy with respect to conversations is lower than if you are in your home. This is analogous to the plain view doctrine. Likewise, viewing real estate from above is not contrary to 4th Amendment. These concepts all relate to expectations of privacy.
As a nation, we have previously held at least our persons off limits to scrutiny by the state, absent fairly compelling reasons. Backscatter X-ray is another close question. I think that the radiation issue will preclude use of this technology, thus avoiding resolution of the 4th Amendment issues. Perhaps nuclear magnetic resonance imaging will emerge as a non-intrusive method to screen passengers.