MOBAs aren't the place for trash talk, IMHO.
In a first person shooter, if a teammate makes a mistake and gets his head blown off by a sniper, its no big deal. I have a better idea where the sniper is now, you think, and there's no bad feelings.
In a MOBA when a teammate is failing, it makes the enemy player/team more powerful, and there may be little the average player can do to stop the situation from getting worse. Most bottom lane supports, for example, don't know anything they can do to stop a top lane player from feeding. So, the natural feeling is one of frustration, that can quickly escalate to anger. Add in people who are intentionally trolling, dual queue players who are from foreign countries and don't speak your language, leavers, and generally toxic players, and it'll wear down the patience of the average gamer until it's to the point where an ADC is cussing out a support for accidentally last hitting 1 minion, not because that's something they'd normally do, but because that frustration level was already too high from the last couple games.
The general gameplay of a MOBA is going to cause normal people to get into toxic behavior pretty naturally on it's own, so this is pretty much the worst place to allow "trash talk" for the sake of regular competitive banter. Mocking an enemy player you've just killed, for an example, might help poor gas on the flames of teammates' attacks, that have already being going on for a while.
As far as the argument of how kids being censored in League going to soften them up for the "real world"... Where the heck is that coming from? You CAN'T insult, mock, or threaten other men IRL, because it presses them into a situation where they're OBLIGATED to harm you, and with women they'll usually have you arrested for being an asshole. For example, go to a bar and start calling people racial slurs. What do you think will happen to you? Competitive sports are no different. You can get away with a little ribbing, maybe, but certainly not LoL style insults, threats, and mockery.