Unfortunately this is another of Bungies' "Infinite Wisdom" decisions. Rather than give people the choice between Vertical and Horizontal they've basically made it so that if you have a non-widescreen TV then you can only play in vertical and if you have a widescreen then you only see it in horizontal.

I know your pain though, just recently despite my TV being a widescreen one Halo2 decided it was going to load in the 4:3 ratio instead of the usual widescreen. Thankfully it sorted itself out but I've no idea how or why it suddenly decided I was only going to get 4:3. It was like having tunnel vision.