But the main reason I haven’t declined airport face scans is actually very simple: I had no idea I could opt out.
It turns out that saying no is not only doable, but surprisingly easy — at least in theory. Everyone, regardless of citizenship, can opt out when it comes to domestic flights in the US. (For international flights, US citizens can opt out but foreign nationals have to participate in face scanning, with some exceptions.) Simply stand away from the camera or keep your face covered with a mask, present your ID, and say, “I opt out of biometrics. I want the standard verification process.”
I don't think that's an option in Korea. I'll look into it. Most people care much less about privacy.