No, it isn't that all of the blame gets shifted onto you. But when you resort to breaking the rules yourself, you stop being an innocent victim of his toxicity, and become a second toxic player in the situation. Granted, almost everyone would agree that what you said/did was in no way as extreme as what he did, but the IFS doesn't view chat logs from the context of what other players were saying or doing. It is only concerned with what you as a player said or did. You are only judged based upon your own actions, which is entirely fair. You wouldn't want to be judged based upon something someone else said, would you? To ensure that the system is entirely free from bias, it looks at each set of logs separately.
And there is a significant difference in defending yourself, and retaliating against someone. For future reference, try to keep in mind that calling for reports on someone, no matter how justified you think they are, is considered harassment.
Btw, this kid was calling for reports on me, but he gets to walk away freely?
See? It bothered you when he did it. Enough that you wanted to defend yourself against it. It's a threatening statement, much the same as a child telling their sibling that they are going to tattle to their parents, or a student threatening to tell the teacher. And just as those situations are viewed negatively, calling for reports in League is considered a form of harassment, and something that Riot does not allow. The IFS only requires a single report for a game to be reviewed by the system, so there is never a need to ask for other reports. Your report is enough. Additional reports, even if the other 9 players in the game report the 10th, do not carry extra weight, and it doesn't make the player get in that much more trouble.
You are allowed to stand up for others in the game. You just aren't allowed to break the rules to do it.