Smile at TSA camera

I was traveling home from the 4th of July and the TSA agent told me to smile for my photo at the tsa camera. Her tone of voice made it sound like it was her way of asking me to stand in front of the camera, so I stepped over and held a neutral (maybe slight smile?) expression for my photo. I travel almost every week for work and I have always held a respectful neutral face for the photo, as I was under the impression that smiling could stretch your face and effect the biometric scanning. When I didn’t smile, her tone changed and she got visibly frustrated. She loudly proclaimed: “I said to SMILE”. Totally confused and a little startled, I said “do these cameras require a smile?” At which point she told me to step aside and cleared me through.

I have never had an interaction like this, but it kind of makes me want to opt out of having my picture taken at all. So, my question is: are you supposed to smile at the camera?