I just got banned for calling a Ryze retarded

So i opened League today ready for another day of slow IP earning and i get a perma ban due to an instant feedback report, i look at the chat and the only negative things i can find from my side aimed at other players are the words 'Retarded' and 'Dumb' towards a Ryze top lane who with all due respect was an idiot that flamed me back, i didn't even swear at anyone, and other players in the match who where flaming didn't get banned either. I wasn't feeding intentionally either, does anyone have an explanation because all the support told me was that these words where 'highly offensive'.