I know it isn't mandatory for every passenger, but it's not going to be that uncommon either. Much like being selected for further baggage checks, it doesn't happen to everyone but it's quite common. If it's something that you are going to be turned away from your flight for refusing to do then it should be made apparent before the purchase of the ticket.Wait wait wait...This isn't mandatory for every person on every flight you know...This one person needed to be scanned for whatever reason, and didn't want it to be done.
If it was something that was done to every passanger (Like the walk through metal detectors) then fair enough...but its not
How many people in the thread have already said 'If you agree to the T&Cs' then it's OK to refuse to let them on the flight. It isn't in the T&Cs though.