Under Terry v Ohio 392 U.S. 1 (1968), law enforcement personnel are permitted to conduct investigative stops, and, should they develop reasonable suspicion, conduct an immediate pat-down for weapons (they are also permitted to seize items immediately apparent as contraband--provided there is no manipulation of the item).
In
Terry, Cleveland Detective McFadden conducted an investigative stop after observing Terry and his comrade, Chilton, walk repeatedly past a shop staring into the shop window. The two men met up with a third man, Katz, several blocks from the shop in question after they had alternately walked past the shop two dozen times. When the three men were together, McFadden approached the men, identified himself as a police officer, requested the men's names, and receiving a mumbled response, frisked the men, taking pistols from Terry and Chilton. The trial court determined that the men were not searched incident to arrest (which is an exception to the warrant requirement), but admitted the firearms as evidence on the grounds that Terry, Chilton, and Katz were acting "suspiciously", meriting additional investigation, and, to protect himself in furthering his (McFadden's) investigation, McFadden was justified in frisking the men.
McFadden made an investigative stop based upon the observation of behavior contrary the norm, and, based upon his years of experience, determined that the men were engaging in, or likely to engage in, criminal behavior. When his suspicions were not mollified by the responses he received, McFadden frisked the men for his protection. McFadden found firearms on Terry and Chilton, and arrested them for carrying concealed weapons.
The major take-away from this, from a law enforcement perspective, is that reasonable suspicion may be developed through an officer's observation of behavior, which he may interpret in light of his training and experience. One might call it "behavioral profiling", which is something law enforcement personnel do every day (although, because of the negative connotation surrounding the word "profiling", the word "targeting" may be used, i.e. "behavioral targeting"). The threat mitigated against in a Terry frisk is from the person, not from the item. A person may carry a firearm or an edged weapon and not be a threat to anyone. It is only when that person's behavior indicates a potential threat does it matter what the person may, or may not, be carrying.
Contrariwise, when the TSA pats someone down, they are not searching for something that might make a person more dangerous, e.g. a weapon, but rather, something that is dangerous in itself--despite the fact that most weapons are not dangerous unless wielded by a person. TSA pat-downs are groping--for lack of a better word--in the dark. There is nothing to indicate a threat, but the TSA will still pat a person down. The fear, apparently, is that people cannot be trusted to exercise good judgment in assessing a threat.
TSA officials are not law enforcement personnel. They do not receive the amount of training that law enforcement personnel receive. Their experience is such that they are not able to develop the ability to gauge a person's likely threat or risk. They are accustomed to working in an environment where perfectly harmless people may act in an aggressive and threatening manner merely because of the headache they are enduring; and where threats--with greater control of their emotions--present no threat at all; or
vice versa. The controlled environment of the airport is such that observation can produce as many false positives, as false negatives, as
true positives or true negatives. It is an environment designed to take the human element out of threat assessment.
The problem with taking the human element out of threat assessment is that it is humans that present threats. Objects--by themselves--are not threats. A well trained human is able to relate to experiences and understand context, which will enable that human to more accurately assess a person's behavior, and the likely threat level it indicates.
Blindly groping everyone who sets off a metal detector creates an environment with so many false positives that no search is conducted with the level of dedication required--it is the "crying wolf" syndrome. People cannot respond with the same level of enthusiasm to repeated false alarms. It is just such an environment which will allow the next deadly terrorist attack.