I'm currently on my way to SDF and was trying to get a shot of the gate departure screen. I made sure no one was in the shot then took it. A gate agent came up to me and told me photographing passengers or employees was illegal. When I asked "and which law is that under?" She said "Homeland Security" and walked away.

I guess she hasn't seen this:

https://www.aclu.org/kyr-photo

Or understand the principal of no reasonable expectation of privacy.