Every country sends it's U23 players (3 senior exceptions allowed) for the Olympics because it's considered a non-FIFA event. For any non-FIFA events the players for senior ball clubs in leagues around the world have to be given approval by their clubs to play in that non-FIFA international event.And the Olympic team was not the real USMNT. The USMNT team can't even do well so how were the 2nd tier guys they sent to the Olympics expected to do well?
