I don't understand this. Clearly a search may be necessary to detect smuggled contraband, but that is the job of Customs. What grounds are there for the TSA to

I don't understand this. Clearly a search may be necessary to detect smuggled contraband, but that is the job of Customs. What grounds are there for the TSA to search someone not boarding a flight? If there is a worry about carrying concealed weapons out of the airport then surely the appropriate agency is the police. Why do the TSA have to do anything with someone who has disembarked and is entering US soil?