The Fourth Amendment to the Constitution restricts government's ability to seizure people. A person is seized when a reasonable person in that situation would f

The Fourth Amendment to the Constitution restricts government's ability to seizure people. A person is seized when a reasonable person in that situation would feel that they are not free to leave. Normally, seizure requires a warrant, but there are exceptions, all of which I think require that the seizing officer have probable cause that a crime has been committed and the person who committed the crime is the one being seized. By asking the police officer if he was free to go, the traveler was triggering this kind of analysis in the officer's head. The officer knows that he cannot seize without probable cause. He can probably temporarily detain the traveler while the TSA investigate the situation, but ultimately, if there's no probable cause, the traveler must be allowed to leave.