The removal of the quickdet script was such a case where Valve stepped in to adjust a bug in the game engine. Playing inside the boundaries of the game engine without cheating. However, it is clear that there are other components involved, like game ethics, behaviour and conduct. For example there is no explicit rule, which forbids to block a door by building a sentry there or to kill a teammate when friendly fire is enabled. Between this behaviour and the way of playing the game properly is a grey area that often enough is controversial and discussed fiercely, like respawn camping, spamming, etc.
Sportsmanship adds another layer to the subject.
It has much to do with the way you treat other players and your code of conduct inside the game. In relation to the other players it means to treat them with respect and to acknowledge their right to enjoy the game as well. But it also suggests a certain level of conduct. This includes refraining from cheap tactics, to keep from harassing other players and to show a little dignity , especially when facing defeat. It also means to keep teams even , definitely in numbers but sometimes circumstances suggest including skill to the equation.