From the Tribunal FAQ:
What's the point of having a language filter if I can get banned for 'adult' language?
We aren't out to punish players who use adult language; in fact, very few players are ever banned for saying stuff like "f***, I missed that skill shot" or "damn, fing close game!" However, the language filter is not an excuse to verbally abuse someone in the game. When you verbally abuse or harass someone by calling them "fing *****" or "ngger noob", that's when you've crossed the line and will be punished by the Tribunal. I've said this before and I'll say it again, we aren't trying to clean online games of adult language; we're trying to reform or get rid of jerks who like to verbally abuse and harass other players.
Why is verbal abuse punishable if there is a mute button/language filter?
Players shouldn’t need to rely on features like the mute button or language filter to engage with other players in positive ways. When a player verbally abuses another player and forces him to use the mute button, they have already created a negative experience for that player.