Just lost a 5v4 because of this, and this is pretty fucking retarded, especially at Platinum, and it happens regularly.
Our top and jungle both alerted everyone they were tilted and going to feed in the lobby, and lo and behold, they had a combined score of 5 kills and 20 deaths at 20 minutes.
They basically weren't even trying; the jungle repeatedly suicided into the enemy team, and the top literally just AFK pushed until the enemy team killed him for the entire game.
This behavior is really not okay. I should **never** get into a ranked game with players who say they're playing poorly and going to feed.