To be honest, I don’t understand your concerns. My experience is that often at airports around the world, all purchasers are requested to show their passport and boarding pass at the register. They are checking that you and the boarding pass holder are the same person. Why, I don’t know but I guess they think there is a valid reason. They simply check the passport photo and name against the boarding pass (all quite visible to you) and that’s it. I hardly think they can “capture some details under the counter very quickly and unobtrusively” .