Dota2 has fixed this by removing the penalty for leaving once another player has abandoned. It's a simple fix and I'm surprised LoL hasn't done it.
One is triggered if there are network connectivity issues, in which case the game isn't scored for anybody. This is to get around the MMR abusers that would queue bots to leave on the other team so they would get a quick win.
The second state is usually triggered if a player has left after first blood. Even then, there are compensatory mechanisms. You lose less MMR than you would normally, and if a player leaves, your team will receive unreliable gold at a faster rate.
Why would anyone do something like that? Every time you played a legit game you'd lose.
Prior to ranked matchmaking, the top 1% was typically around 4.2k hidden MMR. After ranked was introduced, it became closer to 5k. Now you have close to 8-9k MMR, because people are buying 5-6k accounts, and lose until they reach their true skill level (usually somewhere between 2-3k).
The account buyer essentially feeds 3000 MMR to their opponents, which is why a 5k player back when ranked was introduced is now an 8k player.
There are also those who legitimately enjoy griefing dota: their account is probably so deep into low priority stacks that it is effectively banned. Or perhaps they are shadow banned into the bot/multi box pool. So they buy a new account to try and ruin games at high MMR.