Thornton had appealed the suspension – handed down following his attack on the Penguins' Brooks Orpik in a game Dec. 7 at TD Garden – and had an appeal hearing before Bettman on Friday.
Thornton still has the option to appeal the case to a neutral arbitrator. Thornton must make that decision before next Tuesday.
"Simply put, this was a bad act with a bad result, and neither will be tolerated in our game. Again, I note that Mr. Thornton has a reputation for being a “good actor” and a model NHL citizen, but that does not eliminate the need for him to be punished appropriately for his bad act in this case," Bettman said in his 12-page written explanation of his ruling.
Thornton was ejected from the Dec. 7 game and suspended indefinitely. He had a hearing with NHL senior vice president of player safety Brendan Shanahan, who issued a 15-game suspension on Dec. 14.
In his appeal, Thornton did not dispute his violation of NHL rules, but disagreed with the length of the suspension. Thornton had never been suspended prior to the incident, and the NHLPA, which filed the appeal on his behalf, said 10-12 games was a more appropriate punishment.
Thornton has been practicing and traveling with the team. Orpik remains sidelined because of a concussion. Orpik's injury was a significant factor in determining the length of Thornton's suspension.
"We respect the process including the ability to attend the hearing with Commissioner Bettman in support of Shawn. At this time, we will decline comment until the process is complete and Shawn has exhausted all rights available to him," Bruins general manager Peter Chiarelli said in a statement released by the team.