Because the punishment algorithm of the automated system is mostly based on the results derived by the discontinued Tribunal and player's report pattern (if a lot of people have reported someone for saying "kys" it probably means the community in general don't like it), with a few Riot's asserted agenda here and here.
That's why when someone gets banned, it isn't wrong to say it's "as community intended".
.....
Also "kys" is not a defence. It's an attack.
Riot does not counterattacking when it comes to toxicity, on the grounds that otherwise every game will legally end in shitfests. And obviously, no sensible devs would want that; they want "toxicity is bad. The End".