Even though MOBAs are inherent breeding grounds for trolls due to the captive nature of the game, Riot has done themselves (and everyone else) no favors in how they've dealt with things. For a game that was released in 2009 they've done an astonishingly bad job at working from the start to prevent things that _anyone_ who had played multiplayer games in the past 10 years could've predicted.
Even now they seem more interested in allowing and studying the behavior than preventing it. Combine that with their poor-to-completely-dysfunctional matchmaking and the total lack of incentive to even play when your next 20-60 minutes are held hostage by a troll (whether it's on your team, the enemy team intentionally provoking things since forcing opponents to rage or afk is a valid "strategy", or both) and it's hard to take them seriously on toxic players.
Even now they seem more interested in allowing and studying the behavior than preventing it. Combine that with their poor-to-completely-dysfunctional matchmaking and the total lack of incentive to even play when your next 20-60 minutes are held hostage by a troll (whether it's on your team, the enemy team intentionally provoking things since forcing opponents to rage or afk is a valid "strategy", or both) and it's hard to take them seriously on toxic players.