You appear to be missing the point, the ASA will happily allow adverts for violent games (Kane and Able or w/e) but if an advert is very much over the top then they will say "no, change it!". An advert for Resident Evil showing the STARS team blowing peoples heads off for instance, that wouldn't be allowed.

So getting on ones high horse for an advert which the ASA deemed innapropriate when they will allow it modified to meet their rules is a silly horse to get on. The Halo 3 advert didn't show any violence, and is a game just about shooting. Its not about the game itself, its about the advert for the game.