Just thinking about a recent flight out of terminal 4. Why is there an imaging system taking photos of faces?

I mean, we have boarding passes, pass through TSA, passports verified, boarding passes are in hand and scanned.

So does anyone call to question why our faces are being photographed as we board the aircraft?
Is it a violation of privacy of sorts?
And can one, with paid boarding pass on in hand, board the plane but refuse to have their image taken?