Well I think your mom is immature.
In all seriousness, I must agree. I understand people getting mad about one person knowingly and intentionally messing things up for everybody. But for everything else... Well, mistakes happen. Doesn't mean you won't be upset about it, like that 0/8 Jinx leading to a 26/2 Vayne rolling into your base and killing everybody 25 minutes into the game with her full build. A reasonable person will be unhappy about it. The problem comes when, rather than trying to work ways around it if possible, people just get mad at each other and expect things to get better.
What people often don't understand is that in this game, you don't win arguments (at least in the way you want to). Even if you're right, even if everyone on your team and the enemy team knows you're right and if even the "offender" knows you're right, that doesn't change what has happened, and the argument itself provides no real solutions. It is much more productive and beneficial, both for the immediate situation and for possible later situations even in other games, to work out a solution or team-wide change in behavior, tactics, and/or itemization. After all, it turns out that's what team-based strategy games are all about.
I assure you there are some of us out there who are reasonable. We're just rather few and far between.