Diamond Player in Silver game: How is this allowed?

Get a DIAMOND 3 Rengar abuser in my SILVER game. Wtf is Riot's matchmaking? Rito seems to think that if total MMR on both teams is equal, then the game will be balanced. Yet every game I've played where there's a higher elo player in a low MMR game, they either smash the shit out of their lane or out-jungle the oppenent by miles. The difference in skill is so apparent when playing in these matchups. Fix your match making, this shouldn't be happening.