Pictures of the masters.
Depending upon your style of practice, including other martial arts, there are pictures of the masters. We had pictures for Aikido, jujitsu, karate, and judo in the first dojo I trained in. The respect is supposed to be the respect for those who sacrificed, trained, and brought forth the practice we do today. Masters may come and go, but when we bow to the kamiza, or these masters, it is the humble thanks of the practitioners to the masters for giving us their vision of a martial art.
Some of it comes from the old sailing ships that had idols tied to the mast so the gods would protect the sailors. Not to be outdone, the sailors were expected to show their loyalty for ship and king, which could also be country depending on the times. From this praying for a safe journey, and show of respect for king and country, we have the two salutes when we come aboard the ship. One to the officer of the day, who replaces the praying statue, and one to the stern where the ships flag flys.
Now, with the number of people who have served in the militaty, aboard ships, or merely had the experience of saluting to enter or leave a room, the bowing, I must admit, is a cultural salute. It is not improper to bow from the waist, but obviously the human psychi considers it more respectful to bow while on ones knees.
Some people have a problem with bowing to the kamiza, thinking it is a means to program them away from their religion, or that it is a sacraledge, but I say, in a sense, you are reaching for the spirit of martial practice by observing the spirit of all those who have come before you, and those that will come after you by humbling yourself in the acknowledgment that you have emptied yourself of what ever is out side of practice in order to be the best person you can be.
I guess it is no more touchy than changing pictures of masters. It really doesn't matter if someone has an ego about being the master, but the fact that you reccognize within yourself the commitment you have made to practice this art, use it to make yourself and your community a better place, and you will never use it for selfish means, there has to be a love that grows within you for your art and those that ceaselessly worked to give you the continuity of techniques proven again and again.
Maybe, if some military state took over, we would be forced to salute the officer of the deck and bow to the flag of king and county as the common sense is overtaken by lust and greed, but for right now, I do not know of anyone on earth who does not do some sort of bow to silently acknowledge a great feat, or to pay hommage to those who have come before us as we recall their memorys.
Think of it as getting aboard the ship.