The Flag
The whole American nation is represented by the American flag, so it is only right to give it the utmost respect. You will notice that soldiers immediately stand and sometimes salute when it is raised.
Back To The Wall
When you see someone with his back on the wall, it doesn’t mean that he has nowhere to go. It simply means that he is safe. Positioning yourself with your back on the wall means that all you have to worry about are threats from the front. So, you may see ex-military personnel still choose a corner seat with the wall behind them even in their regular civilian life.