I should be allowed to defend myself and not get banned

No this never happened to me, I have never been banned. This is a situation that happened previously to a cool dude I found in Solo Queue. We were in game, he was playing as our mid lane was getting angry at him for not ganking. The Fizz died 6 times during the lane phase because of the enemy jungler and he said it was our amumus fault for being bad and not ganking. Amumu told him that it was his fault for not warding, and that he wanted to spread his power elsewhere. Fizz kept fighting him about it, even intentionally not helping with Dragon and saying "Why should I help you, you didn't help me!". They dissed each other back and forth until I told Amumu to mute him, which he did. After the match I befriended the Amumu, only to find out he was Chat Restricted??? If someone is defending themselves, why are they in the wrong? Many people are taught at a young age to not let others push you over, so of course this would transfer online. There is no reason that anyone who is defending themselves should get punished. I rarely post on this forum in particular, but this made me very angry. Regardless if the Fizz got punished or not, Amumu did nothing wrong here. That's all I wanted to say.

-End Rant