It's because people view it as bad sportsmanship especially when you upload a win against someone you lose to consistently. But yeah, I do agree that people are too easily offended by that. As long as there's no trash talking, all is good.
I'm sorry but... what? Do FPS people upload fragmovies of them being headshot? Do MMO players upload fraps of their failed raids? Do professional LoL players upload how they failed terribly?
Is this real life?
People need to grow up.