The checkpoint is a public place, in a public building. Pictures are allowed. To my understanding, the only thing that isn't allowed to be photographed are the images on the monitors (like X-ray images, etc.) that are there.

If I'd been that person, and if the TSO had caught me, I'd have refused to allow access to the photo. I may have even denied that I was the person who took the picture. By the time they would have the whole thing sorted out, the photo would be uploaded to somewhere else, so who cares if it gets deleted from the device...

Perhaps I'll add this to my list of "must dos" near a checkpoint. It seems as though snapping a photo or two might anger TSOs even more than comments about the nonsense that happens at their stations.