A lot of people act like it's just bad luck that League of Legend's player base is so abusive, or they say things like "Isn't it too bad that MOBAs attract such a bad group of people?" I hardly ever see the community blamed on the game itself, but you're talking relatively high stress game where you invest a significant amount of time and where victory means something in terms of unlockables and ranking. Further, a poor teammate not only fails to help you win but can actually aid your enemies. The game creates an environment that's just asking for abusive behavior from stressed out, frustrated players.
It's entirely possible that this stress will come out from players in real life as well as in the game, but I don't think it's safe to jump to any conclusions about whether it will happen more if they're allowed to vent more in game. What you could do if you wanted to address that is give them a little cool down period after a match... Meh. Or just get them to play a different game.