Rather than being allowed, it's more like what scazzman said, it's impossible to prevent, stop, or prove with convincing evidence. That is why it is discouraged but not bannable on riot's end.
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The reason why I am in bronze is because I belong there. That is my skill level.
Coincidentally that is the exact reason why I gave up my main account and started a new one, or "smurf" as you may say.
My skill level is bronze V~III at best, but because my 10 placements got autofilled as support and I got carried to gold V, now I have to lose hundreds or even thousands of games (being stomped hard at it, if I may add) before I'm finally put back to where I belong.
Before that happens though, I am going to suffer. I cannot lane against anyone even if they have 1337ms, and if I main support I will feed. If I knew I'm going to get stomped for the next several hundreds or even thousands of games, regardless of win or lose, it is straight up discouraging to continue playing. Hence I'd rather start anew and hope I'll be matched with those around my level.
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What do you smurfs want to even prove?
You've forgotten this is LoL na community we have here. This player base in particular loves to see others suffer - to the extent where they'd run straight to feed, then ask their teammates how does it feel, following up with ping spam (noted, there is no prior argument and they've never met each other before, the game just started, there was no reason nor cause). This is a community that finds delight from stranger's sufferings, and smurfing to make weaker opponents suffer while unleashing the trash talk is one of the most efficient methods at it.
