Thornton was ordered to serve a 15-game suspension for the fight with Pittsburgh defenseman Brooks Orpik. Thornton knocked Orpik to the ice and Orpik was taken from the rink on a stretcher and hasn't played since.
He has passed the NHL's concussion protocols, however, and may return to action Friday when the Penguins play Carolina.
Thornton appealed the suspension, the strongest penalty issued this season by the NHL Department of Player Safety head Brendan Shanahan.
"In this case there is no dispute that Mr. Thornton's conduct was in violation of Rule 46.15 and warranted a match penalty," Bettman said in a statement issued Tuesday. "The NHLPA does not dispute that a lengthy suspension was warranted. The only question raised by this appeal is the appropriate length of the penalty, which the Union contends should be reduced from fifteen (15) games to somewhere in the range of 10-12 games.
"I find that there is clear and convincing evidence to support Mr. Shanahan's conclusion that a suspension of fifteen (15) games is appropriate."
Thornton can take the additional step of submitting a grievance to an independent arbitrator.
If the 15-game suspension is upheld, Thornton will forfeit nearly $85,000 in salary.