Originally Posted by wildcatlh

LEOs don't have the right to question absent reasonable suspicion, as defined in Terry v. Ohio (subject to a narrow exception for state laws that require you to give your name to law enforcement officers upon request). (Clarification: They do have the right to question, but you're under no obligation to answer and are free to walk away at any time, and your refusal to answer is not grounds for reasonable suspicion and a custodial stop)

The proper constitutional answer to your question though, is, "nobody". That being said, considering the exceptions to the constitution that the TSA has given themselves and the courts have given police to "fight the drug war" (to the point where the 4th amendment more or less no longer exists in drug arrests), it seems to be "whoever feels like it".