Fred is right, but it is also has to do w/ fairness. For example, lets say the US beat Ghana before the Italy-Czech Republic game. In order for both Italy and the Czech Republic to qualify for the next round, all they would have to do is draw. Therefore, they would play for a draw and not attack each other at all. However, there is no way to completely prevent this. Case in point Euro 2002, when Danish and Swedish players conspired with each other to play for a draw during half-time when they heard that Italy was defeating Bulgaria by a large margin. They didn't attack each other's net for the entire second half, and thus both moved on to the next stage due to goal differentials.