Oh, I can conceive of many other answers!

1. Perhaps the location of the "stop" button is SSI -- or even double-special SSI, and nobody at DCA knows where it is.
2. Perhaps the "stop" button is located in an area so secure that TSA staff are not permitted to enter it.
3. Perhaps the "stop" button is wired into the same electrical circuits as the metal detectors and x-ray machines, so that stopping the video would shut down screening as well.

I could go on and on...but I'll spare you that!

Bruce