In soccer tournaments, goal differential determines advancment if there is a tie on points. If the US loses to Sweden or even ties for second place, they could potentially not advance or lose the first seed if they call off the dogs and don't have the goal advantage. With that possibility, you would be nothing short of stupid to not go for every goal. The US would be arrogant and dumb to assume they will definitely get the most points and so don't need to worry about goal differential.