Smurfs

The only viable excuse that allows Riot to let it happen without banning smurfs who boost low-ranking friends (when they say it in chat to provide proof) is that, supposedly, it's just as likely that you'll get that duo on your team than not. So in essence the equal-odds randomness makes it fair.

But that's mathematically incorrect. The fact you yourself are playing the game without a smurf buddy makes it so that you are always more likely to encounter enemies doing so than allies.

The explanation is actually pretty simple:

• You have 4 allies. We'll label them 1 2 3 4, possible duos are 1&2, 1&3, 1&4, 2&3, 2&4, and 3&4. That's 6 possible pairings of an ally getting pseudo-boosted and a smurf.

However,

• You have 5 enemies. 1 2 3 4 5, possible duos are 1&2, 1&3, 1&4, 1&5, 2&3, 2&4, 2&5, 3&4, 3&5, and 4&5. That's 10 possible pairings.

It's almost twice as likely you'll encounter an enemy smurfing duo than an ally one, unless you're a part of a duo.

Granted, Riot cracking down on smurfs would take a chunk of profit from them, so they have more incentive to let the unfair practice continue than to make the game fair for solo players. So we'll likely not see anything done about it. I just wanted to get the word out that it is in fact skewed towards your enemies.