I think it is talking about how in life, sometimes you must make tough decisions that you or other people will not like, but they are necessary to be fair (or just). And often what we consider to be 'foul' is the fairest way. For example when a lowly ranked sports team must beat a highly ranked team to be able to compete in the World Cup....the lower ranked team will be beaten by a large score. This is foul but fair (every team must earn their right to go to the World Cup).