You are sort of correct that neither is your responsibility -- you do not have to protect someone's feelings, and everyone can benefit from learning to control their emotions better.
However, is it not OUR responsibility to protect you from the consequences of your actions, which have been clearly laid out by Riot. It is not OUR responsibility to cater to your inability to behave in a civilized fashion by saying that you're free to say whatever you want without consequence. It is not our responsibility to sit quietly by and listen to every form of abuse you wish to spew forth.
Whether we're offended or not is immaterial. Personally? I find most flamers kind of adorable, since I can brush it off easily and that tends to frustrate them. That still doesn't mean I want them as part of my community.