I think it's fair.
You can pin a player to the board in a physically over powering manner.
You can use your body to physically knock him off balance and move him from your space.
You can not use force in an unnecessary manner to throw a player down to the ice, after he no longer has the puck for that matter. That's roughing.
Also this isn't an Ales Hemsky vs Robyn Regher deal.
The league has changed and Pettersson is looking like he may be a (potential) superstar and Calder favourite right now. Good for the league for standing up for young skilled players getting deliberately injured by nobodies.
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