I blame both.
But I'm also smart enough to know that someone drunk and hitting my car can't really do much but possibly damage my car a bit. But, if I get out of my car and confront an unknown person on the side of the road, I can be seriously hurt, killed, or my passengers could be seriously hurt. It's not worth it to me.
In this case, the drunk guy was killed and the he-man coach will have to live with that the rest of his life.
I can live with some damage to my car (if even that). I have nothing to prove to anyone else by getting out and confronting someone.