You may think that it's absurd, but Riot and the overall community doesn't. Riot has repeatedly stated that the mute button is not a "harass someone for free" button, and that jerks are the ones that ruin the game. You talk about how people that afk, troll and feed are not playing the game, but one could also argue that the people who spend allot of time raging and flaming in the chat are also not playing the game either, as they're spending the majority of their time thinking of exactly how they can rage and flame at someone effectively (effectively to them). Regardless though the point is that it doesn't matter what you're doing, if you're not trying to create a neutral or positive atmosphere then you're doing something wrong. Besides, people generally have more fun and are also generally more willing to cooperate with others as well when there's not someone going out of their way to be a jerk.
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.
From Understanding the Tribunal:
5) Low skill players should be banned from the game! The Tribunal should only ban Leavers/AFK and intentional feeders, those are the only types of players that really ruin games!
a) No. Just no. Jerks ruin games. League of Legends is for everyone whether they are 500 Elo or 2500 Elo. It's not a game for jerks.