1. The TSA is legally entitled to deference of its interpretations of its own regulations. See Chevron (for statutes. Yes, they get deference to interpret the statutes congress passed that say what they are allowed to do), or Auer (for their interpretations of their own regulations).
It's ridiculous (in the case of Auer deference), but true.
However, at least right now, if the TSA says "our regulations mean X", courts give them deference that their regulations mean X, and it takes a lot to overcome that.
There is at least some hope that Auer deference will die in the next few years. Humorously, one of the justices most people here seem to hate the most, Scalia, is now the strongest opponent of Auer deference:
"For decades, and for no good reason, we have been giving agencies the authority to say what their rules mean, under the harmless-sounding banner of “defer[ring] to an agency’s interpretation of its own regulations."
(and earlier) "The Court there gives effect to a reading of EPA’s regulations that is not the most natural one, simply because EPA says that it believes the unnatural reading is right. It does this, more- over, even though the agency has vividly illustrated that it can write a rule saying precisely what it means—by doing just that while these cases were being briefed.
Enough is enough."
(You can also look at his earlier Talk America concurrence on this subject. http://www.volokh.com/2011/06/09/justice-scalia-questions-au... is a good reference about it)
2. Davis, which he tried to quote, says mostly exactly the opposite of what he wants it to:
"airport screenings are considered to be administrative searches because they are conducted as part of a general regulatory scheme, where the essential administrative purpose is to prevent the carrying of weapons or explosives aboard aircraft."
Note, this seems innocuous at first to an outsider, but administrative searches are not subject to the same requirements as general 4th amendment searches. They only have to be reasonable, not have probable cause. In particular, later it gets you down the path to this: "To judge reasonableness, it is necessary to balance the right to be free of intrusion with “society’s interest in safe air travel.” [US v. Marquez, 410 F.3d 612]). In a fight between "society's interest in safe air travel" vs "justifying why you need a liter of juice", i know who i would bet on. Again, i think such legal rules are wrong-headed, but it doesn't change my view of the current legal state of the world.
But back to US v. Davis:
"[an administrative search is allowed if] no more intrusive or intensive than necessary, in light of current technology, to detect weapons or explosives, confined in good faith to that purpose, and passengers may avoid the search by electing not to fly." This is a zinger for two reasons:
1. Now, remember, the TSA, because they get deference, mostly gets to decide what is "more intensive or intrusive than necessary", and courts will defer to them, for the most part.
2. As long as they let you out by saying you won't fly, they pretty much get a free pass.
So there you go. Sucks but true.
PS The idea that searches performed by the TSA are automatically administrative, or even that there should be an administrative search exception, was also conjured up out of whole cloth by the courts, so expecting them to suddenly reverse course and apply "the full force" of the 4th amendment (god i wish i didn't have to use that phrase), is well, wrong headed. Barring the supreme court getting a grand urge to stake out a position here (which is rare), if it happens, it will happen when society is less accepting of this kind of thing, not when it is more.