I just saw it in one of my games today: 4 Platinum, 3 Gold and... a Bronze 3! That is just not possible without him being in a party with somebody else.
And what did the Bronze guy do? He didn't move during the 1st carousel until everybody else grabbed something. Then he grabbed something that was left there. During the minion rounds he got the standard 3 Woodland combo, bought lv 4, added a Renekton to it and... went AFK. He didn't disconnect, just stopped doing anything else. Never upgraded, never added new champs, didn't even pick up the loot from the crags. Of course, he got killed 1st. But even then he didn't exit until one of the Platinum players got eliminated in 4th place. Then they left at the same time.
It's totally obvious that the Bronze player and the Platinum player were the same person playing from two accounts who created a ranked game with a party of two.
Why is this allowed??