Are you aware that horses are often times lined up head-to-tail in halter classes to let the judge see every horse from the side at once? What do you do when this happens; walk up to the judge and tell him you can't line up like the rest, or take a chance and let another horse get kicked?
Also, on the note of not teaching non-kicking manners because riders are supposed to keep horses away from other horses' backends. That is like saying that because drivers are supposed to stop at stop signs, you should never look both ways.
I'm not saying that you shouldn't at least TRY to train your horse not to kick, but let's face it, it's something that they DO, so it's safer to assume that every horse kicks. I know that the assumption that my old horse didn't (because I'd told them he wasn't a kicker, because he'd never kicked before), actually CAUSED him to become one. And he was real bad, he would swing around and plant a hoof on any horse that came within range in any direction. Took a skilled rider to keep him from doing it, and there was nothing you could do to teach him not to try. He had issues and was very aggressive with other horses by the end. There was NOTHING you could possibly do that would stop that horse from attacking other horses. If he wanted to, he was going to, unless the person riding him had 50 years of experience and was wearing spurs to keep him from spinning around. I'm not 50, so obviously I don't have that kind of experience, and I don't wear spurs.
I guess I see both sides here - the owner of the kicker (CAUSED by horses up his ass might I add), and the owner of the non-kicker who hates kickers. I've been kicked before by a horse aiming for the pony I was riding - caught my foot and nearly broke it. Am I lucky because my horses have never been lamed by a kick? Maybe. Or maybe I'm SMART because I don't let other people's horses get that close.