False correlation. Premades see less toxic behavior because they're usually all friends, or at the LEAST because they're likely on skype or something and so they're not using the chat at all. They could be cursing and flaming each other all game, but the chat would be pristine.
Premades however, still abuse their non-grouped teammates. What's more, FORCING players to play in groups isn't the answer. It's a stupid thing that is ruining the game for anyone who prefers to play solo or who doesn't have a pile of friends who play the game too. Sure the 19 year old bro has like 10 other guys he knows who play, but the 26 year old guy with a job and a family doesn't.
Imagine WoW during it's heyday releasing a patch that gave parties and raids +20% damage and health per person in their group... then balanced the monster stats around the inflated stats that they knew people would have due to this bonus. The people grinding the game solo for fun are utterly RUINED now, because they are at a severe disadvantage against everything they encounter because they cannot or have chosen not to play with others.
That's such a stupid idea that you KNOW it would never happen. So why is Riot introducing things that give MORE advantage to certain players? Having a group in skype or whatnot with you is already a big advantage over an uncoordinated solo team. By allowing those groups into ranked they are basically saying "If you don't have a premade in ranked, you deserve to be playing at a disadvantage. Even if BOTH teams have one, the solo players on each team are still left at the kids table to pick over the scraps while the people playing the game "correctly" decide who wins or loses.
Riot massively dropped the ball here, and well-intentioned or not they should have seen the downsides of their actions before they made them mandatory. Solo players shouldn't have to suffer for months on end just because they wanted to try something out and they're too short-sighted to have predicted the outcome.