The incident, which resulted in Orpik sustaining a concussion, occurred during the first period of Boston's 3-2 home win against Pittsburgh on Dec. 7 after Thornton pulled the unsuspecting Orpik to the ice.
Thornton then punched Orpik several times in the head before he was assessed a match penalty, ejected and immediately suspended.
"This cannot be described as a hockey play that went bad, nor do we consider this a spontaneous reaction to an incident that just occurred," the NHL said in a video link posted on its website.
"Rather, it is our view that this was an act of retribution for an incident that occurred earlier in the game. The result of this action by Thornton was a serious injury to Orpik."
Media reports after the game said Thornton was angry because Orpik's hit to Bruins forward Loui Eriksson during the first shift knocked the Bruins forward out of the game.
Thornton, who had a hearing with the league's player safety department in New York on Friday, expressed remorse for his actions.
"I feel awful," the Bruins forward told reporters after the Dec. 7 game. "It wasn't my intention for that outcome. I know Brooksie. I've gotten to know him over the last seven years here.
"I skate with him in the summer and through the lockout. I've texted him a couple times. It's not what I wanted to see or what anybody wanted to see."
Under the terms of the collective bargaining agreement and based on his average annual salary, Thornton will forfeit $84,615.45 which will go to the players' emergency assistance fund.