They even acknowledge that Mantha instigated the confrontation and that Buchnevich was unwillingly put in the position of defending himself. I guess the NHL's position is that he should have just let himself get crosschecked in the neck.
"It is important to note that while we agree that Buchnevich’s actions are defensive in nature, players are not excused from following league rules because of the actions of their opponent. While Mantha’s actions may have been provacative, Buchnevich is not permitted to respond to that provacation with a reckless and forceful strike directly to the face of an opponent.