But tackles are allowed, biting is not. If someone uses tackling as a means to another end than winning the ball they should be punished and usually are. The severity of the damage caused shouldn't matter, it's the act that is punishable.
Some players are reckless tacklers, like Clattermole who has and will continue to injure opponents on occasions despite suffering long term injuries himself. He's just stupid and more should probably be made of cases like him, but the rough, hard tackling midfielder has always been a part of the game.
A cynical, cheating, diving taco like Suarez is a new thing and a special case. Every time he does something it will be blown up even more because he has previous. Other players who get into one-off situations of poor behaviour will be let off far more eaily.