so why am I still being punished for someone disconnecting 4 minutes into the game and not returning

Because you'll more often be rewarded for someone on the other team doing the same. RNG, embrace it.

Riot is too busy trying to censor naughty language on the internet to take care of things that actually do ruin the gaming experience. Bad match making, trolls, afk's, scripters, hackers, etc...

I don't know ask the person that left.