I don't think anyone in the US really cares about metal detectors, humans don't naturally contain metal and it is done completely hands off with no extra visual or biometric information or saved data. Plenty of people in this thread who opted out of other security measures still walked through a metal detector without any special note. Court houses and police stations have often have metal detectors that even a Senator or President would have to walk through. The same cannot be said of direct imaging of your body though or facial recognition or anything. If you wouldn't put your children through the process to go into school each day then it seems completely bonkers to require it for any form of mass transit.