You can't jam a dead guy into a wood chipper and sneak away with a PG-13 rating. That's just not the way the MPAA rating system works.
And there are plenty more instances of shocking violence: a state trooper's blood spattering on Carl's face, the cold-blooded murder of a cowering young woman, the senseless murder of a parking attendant, and the kidnapping. There's some explicit sex in the movie besides, though it's appropriately weird. So yeah: this movie puts the "R" in Fargo.