The facial scanners are only used on international flights. Now, realize this:

1. They already have your passport info including photo. The facial scan uses your passport photo as a point of comparison
2. There are cameras all over the airport. By the time you get to the gate, it's too late. They already have your face from multiple angles. They can check the TSA data from when you passed through the checkpoint to identify you.
3. As others have mentioned, you are in a public space. There is no expectation of privacy when you are in a public space.