The_Emperor said:
Actually, I can answer this as to why. When we had Microsoft come to our school to essentially market XNA and the XBLA, someone inquired about using avatars in games, and the rules to them.
Please allow me to paraphrase, as it's been a long while. Even though many Xbox games are marketed to "grown-ups." There are still people below the age of 16 that play, and have gold accounts. Microsoft can attempt to control this, but your never going to be able to tell if someone's 23, or 13. (How old am I?)
There is to be no violence, and no aggression from, or against Avatars. Maybe something funny like getting dizzy or falling over would be acceptable, as well, there's no real harm. But anyone who is, or acts below the age of 16 might not overly like their virtual self vomiting, or getting chain-sawed in half, or gibbed to bits. I sort of agree. The Avatars are supposed to be fun, not slaughtered for shits. Because you just know, someone who's 25 is going to take personally, or some parent's going to shit a brick when Virtual Timmy is shot and killed.
TL;DR, Avatars are supposed to be friendly representations of yourself. Not part of any bloodshed, because you know someone's going to take it personally. Guns are just plain associated with bloodshed.