It seems to me that lowering one’s head to allow someone to put a medal around your neck is still bowing. It in effect signifies that you are honored and humbled by the award you are receiving, and that you recognize and respect the authority of the person who is giving it to you. In effect, “I recognize your authority to decide whether I am or am not worthy of this award.”

On the other hand, I’m no expert on etiquette. Miss Manners says never to bow to a foreign monarch, even if you’re an ordinary non-president citizen, because the bow signifies that you accept the monarch’s power over you.