Well, in the States spitting on someone is considered assault and battery (usually, however, such charges mostly occur when it's someone in law enforcement, emergency response, or fire response who is assaulted). It's also dangerous, and if a person is found to have a communicable disease the charges can even be worse. Not sure how it's considered in other countries.
I think it's a fair deal because, while there are plenty of cheap shots taken during the game, they actually are still just aspects of the game taken to an extreme. Stamping, stomping and kicking all legally occur during play, so it's somewhat understandable that sometimes a player lets his frustration or aggression get the better of him and stamp on a player instead of the ball. NOT saying it's right, just saying that I can see why it happens. Eye-gouging is horrific and should be punished to the full, but I think there is often (though not always) the question as to whether the gouger intentionally went for the eye, or was just caught up in the heat of the moment. Still, all these are offenses that happen due to the sheer physicality of the game, taken too far.
However, spitting on a player is not part of the game, in any way, shape, size, or form. It's disgusting, and a purposeful action meant to humiliate, to make a public show of disdain against another human being. It is, more than anything else, an insult to a player, to a team, and ultimately to the sport. Give him a slap on the wrist and the dignity of the game goes right to hell. Other players will feel that they, too, can get away with childish, petty, insolent behavior. This penalty sends a very clear message that players cannot and should not degrade or humiliate other players on the pitch. I'm actually pleased with this.
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