Why do people have to keep their hand "on guard" when you open the door for them?

I was reading JohnPowell’s comment in the “thank you” question that reminded me of something that has always saddened me about the psychological condition of America.

Whenever I open the door for someone their instinct seems to go on the defensive and keep a hand up as though protecting themselves from the possibility of me letting go of the door right at the point of no return so it slams into them.

I believe this is because of a lack of trust our sub conscious has developed because of the world around us, your thoughts?