I think you (and a lot of others) need to watch the video again. When Ben toe drags, his left shoulder moves forward relative to his head.
... blah blah blah.
Addendum: Shanahan explains, that his (Ben Smith) head position did not significantly change. I would like to point out that his head position did change in relation to his left shoulder, which was the target. His head was protected by his left shoulder until he dropped it back and opened his body allowing the natural and pre-chosen trajectory to follow into his head instead of his shoulder. He still put himself into a vulnerable position by moving his shoulders.
Brendan took a bad line to the hit. The suspension does seem a bit harsh, but hopefully he'll learn from it. Not just to avoid penalties/suspensions or to avoid hurting people but to be a better player. A few people pointed out that the hit was just a couple inches from being a good shoulder to shoulder check. But it was also a couple inches the other way from blowing by him altogether and leaving Ben with a clear break on goal. Brendan actually had to lean in a bit to make contact.