Also lets not forget that this game can be played by more than 2 people. I know a lot of us prefer no allies, but some people want 6-7 players. In that case, who would want to be the US? If lucky you can fight after round 5 or 6 or whatever? If you draw US you might as well go home and come back in a few hours. Call me when you attack pearl!
I like having the US come into the war at a defined time, and not too late because this gives the US player something to do. I think this is not the place to search for balance.