The problem is that the ball was still in the air when Burfict hit him. I would agree that it wasn't a dirty hit (mean - yes, vicious - yes, but not dirty), though it should still have been a penalty at that point of the play.
Burfict went shoulder-to-chest. It does look like their helmets made a little contact (which is unclear with the camera angle), but, even if there was incidental contact, it was clear that Burfict was not targeting the head or neck.