So I was ranked about 4th in silver league a few days ago. I do my placement and I lose to a 6 pool on a new map and get placed in 50th bronze. Maybe I'm crazy but that is totally unfair. Has something like this happened to anyone else?

The reason you got placed into Bronze was because your MMR was that of a Bronze player. It means that you could have been demoted anyways if the season didn't just end. Did you go on a really bad losing streak before the reset or something? 
 
Just keep playing and you should get back up into silver.

I had been winning at least more than half my games I think. But yeah, I guess I'll just have to work my way up again.

A kinda similar thing happened to be, though I was in gold league, won my placement match and got dumped into bronze. I've been facing mostly players who have only played a couple of games, and I haven't lost yet.

I am totally not a top tier player, but every time I beat someone who appears to be struggling with the core mechanics, a part of me dies.

You're exactly where you need to be.

I dunno, I don't think I'd mind getting demoted. Would be nice stomping people for a while instead of having each game be a sad, stressful situation.

The problem is no one really understands how the ladder system works. Not even Blizzard... 
 
plus the season/ladder needs to level out. I drop 30 ranks over night. Win two games and climb right back up.