UPDATE: League explains why Shazier hit legal

[oncemoreuntothejimbreech]

Well, I disagree with the "lining up" explanation.  Gio wasn't moving when he was hit in the face mask so there wasn't any "angle" involved on his part and there is no way he could have avoided contact.

http://www.nfl.com/videos/nfl-videos/0ap2000000152145/Crown-of-the-helmet-hit-rule-explained

All of that is a moot point, because unnecessary roughness Article 6i:  "using any part of a player’s helmet or facemask to butt, spear, or ram an opponent violently or unnecessarily."