Part of it is the game. Rugby's an exceptionally physical contact sport; if you play it you expect to get hurt. You also expect to try to hurt the opposition. And then you all go and have a drink and meal together afterwards and, in the higher levels of the game, make sure you swap shirts with your opposite number.
Soccer is meant to be minimal contact, but you keep seeing the cheats pulling shirts, elbowing, etc. and the referees do nothing about it even though they see it. Quite frankly, if the officials refuse to uphold the laws of the game, it is hardly surprising that a disgusting ethos of cheating surrounds it.
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Stephen