They probably don't want people listening to their "security" conversations.
"Checkpoint 1! We have a water bottle here. I repeat, a water bottle. Volume exeeds 100 ml. Over!"
"Last names A-M, go on break now. N-Z break will be in 1 hr."
"Need gate harassment teams to gates 1 - 10, inclusive. Let's get frisky, people."
"Gropey, this is SPOTter. Suggest a 'secondary' for the tomato in green 2 people from the WTMD."