They are legitimate since implicit rules are important too. If we all play as though the rules of sportsmanship are being observed and someone does not, then he or she gains an unfair advantage and others can rightly object. It is all a matter of establishing an even playing field, and if there is not general agreement on how the game is to be played, then at least we know what to expect. Once there is such agreement, the occasional malefector will have an advantage over others, something which will itself often be dealt with on the sportsfield in an informal and painful way!
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When playing games, be they sports, board games, video games or what have you, there are almost always fixed rules, and violating these rules is usually accepted to be illegal within the game. However, there are also often informal rules of good sportsmanship and fair play which would prohibit certain kinds of behavior. However, such informal rules aren't explicitly a part of the game, and it seems that violating them is still within the rules of the game. If a person participates in a game, expecting good sportsmanship of some kind or another, but is instead treated to a game where their opponents demonstrate poor sportsmanship, is it within the player's rights to complain about that poor sportsmanship? Has the player been wronged? It seems that on the one hand, in consensually participating in a game without explicit rules of conduct punishing poor sportsmanship, the player has set themselves up for a situation in which they have no right to complain. On the other hand, it seems entirely reasonable for a person to expect good sportsmanship on the part of all players involved (unless the individual players are known for poor sportsmanship, but let's assume that isn't the case). So how legitimate are complaints about poor sportsmanship?