Because what Chara did would be acceptable anywhere else on the ice. Well I mean, it's a penalty yes, but still acceptable.
It's just a matter of the arena design that caused his typical hockey play to become a serious injury.
IMO, the league needs to suspend for plays resulting in head trauma to cut down on it. I don't agree that all instances involving head injury are deserving, but it needs to be cut and dry. They cannot continue to rule based on what they believe the "intent" is, because it's fucking impossible and utterly stupid that they do it.
Chara should be suspended because of the head injury, but not because of what he did. However, this is my stance and not the leagues, so I understand why he isn't.
I don't agree that injuries should define suspensions, so this is a bit hypocritical, but there doesn't seem to be any other solution that will make the players show more respect on the ice (not that I feel this instance was disrespectful, just bad luck)