Everyone has bad games.
Feeding in one game doesn't make someone a bad player.
Likewise, just pointing that someone fed one of the enemies is not calling that person a bad player.
It's not negative attitude or verbal abuse. It's simply stating that in that current game champion X died to champion Y several times.
(It's the same as if you said an enemy fed your ally. If I'm wrong and the former is an offense, then so is the latter.)
Please note: "Tristana fed Vayne." is entirely different from "Tristana sucks, and should uninstall because she fed Vayne."
One is a comment on #game state. It lets your other allies know that if they try to 2v1 alongside Tris against Vayne, they may still lose the fight.** Fed is a very common term for these games and is used to describe such a situation. Saying just the Vayne IS fed doesn't give enough information for your allies to know Tris won't always be able to make the 2v1 a win. **
The other is harassment-which may accompany the first, but does not change the meaning of the first. #Please know the difference.
And you can't pretend that a phrase which isn't negative is against the rules when it's misinterpreted. If you were to say the enemies are at Dragon, and the jungler happens to be too far to contest, would you expect the jungler to report you because it "made him feel bad"? That holds just as much imagined hostility as the other scenario.