I'm not sure if this is a restriction of the hl engine, I can't remember if TS did it though so don't quote me on it.
But in the end, it's more of a team and gameplay stand point. If you want 360 degree BFP has it. And it's fun, but it really serves no purpose.
Having every player Upright at all times makes gameplay easier, and more believable to the show. A good 98% of the fights were upright. 360 would be fun, but it hinders the gameplay and adds too many variables that we just don't have the time, and more honestly willingness to deal with.