Checking for weapons or explosives at an airport concourse is not considered to be unreasonable search and thus not protected under the 4th.
It IS unreasonable on multiple counts:
1. "Unreasonable" in the 4th Amendment referred to performing the search with no apparent reason to believe something would be found, and no indication that a crime was being committed. In practically all cases, there is absolutely no indication whatsoever that anyone entering the "sterile area" of an airport is associated with any past, present or future flight-related crime - it is, therefore, legally unreasonable to search them.
2. Considering the 2nd Amendment right to keep and bear arms ("...shall not be infringed", end of declaration), arbitrarily searching someone for something they have a right to is inherently unreasonable. See the latest "Emerson" and "DeCSS" court rulings for supporting legal views.
3. By your reasoning, random or pervasive searches anywhere else in society would be "reasonable" - but courts regularly deem far more "reasonable" searches unConstitutional. A guy wearing gang colors and obviously carrying a small heavy item in his pocket in a drug-dealing area cannot be searched for guns/drugs by police (despite obvious high probability of serious crimes past/present/future involved) and such a search, without corroborating evidence, would be ruled "unreasonable" - but an average person just stepping onto an airplane can be searched for just being there...this is "unreasonable" by any reasoning.
4. An otherwise baseless search performed to find items deemed "weapons" arbitrarily by an agency and not by explicit legislation is unreasonable. My right to be secure in my person & effects cannot reasonably be suspended/violated just because a faceless unelected burrowcrap, unaware of my existence, arbitrarily decided that I can't be trusted with a nail file.
5. The 4th Amendment REQUIRES that any search be predicated by probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized. Translation: a search may only be done upon a SPECIFIC person, with a SPECIFIC reason to believe crime-related items are possessed, with SWORN testimony to a judge, with SPECIFIC declaration of where the search will occur and what is expected to be found. Arbitrarily searching billions of passengers, especially when questionable items are RARELY found (and most of those otherwise legal without clear criminal intent), is almost as pure an example of "unreasonable search" as can be given.