If I was the lawyer for the TSA, I would argue that we don't want you to take pictures of the monitor because you might see something which is private to the individual whose luggage is being searched. That is, when Pasenger XX goes in public, with items inside a brown paper bag, you can't photograph what is inside the brown paper bag. And, even though it is being carried in public, you can't insist that Pasenger XX open the brown paper bag for you to take a picture. Now Passenger XX goes to the airport and puts the brown paper bag in the machine, and the monitor shows that Passenger XX is carrying sex toys. The argument could be made that you are invading the privacy of Passenger XX when you photograph that monitor picture.
HOWEVER, the couter-argument is "If I can see the monitor in public so can other people and the argument about privacy no longer works. Hide your monitor so that no one can see it and we don't have a problem."