Perhaps, but getting bullied in school involves a constant daily threat and the possibility of physical harm. In league, you're dealing with people who you'll probably never see again, and you can mute them if you want. The necessity to "defend yourself" from internet trolls is infinitely less present than with school bullies.
Plus, lots of people that flame in League aren't bullies. They're just people that don't know how to properly communicate. Stooping to their level causes games to spiral out of control, with even non-participating teammates getting dragged into flame wars. The only sane response to flaming teammates is to ignore them, or to de-escalate the situation if possible. Reactive flaming would be dumb even if it weren't punishable, and ignorance of the fact that it IS punishable can't be claimed for a perma when the individual clearly went through several stages of punishment to get to that point.