Because thats a penalty to for a goalie to take his mask off intentionally. The rule below the Kings had the puck.
The NHL rulebook covers helmets in 9.6:
When a goalkeeper has lost his helmet and/or face mask and his team has control of the puck, play shall be stopped immediately to allow the goalkeeper the opportunity to regain his helmet and/or face mask. When the opposing team has control of the puck, play shall only be stopped if there is no immediate and impending scoring opportunity.
When a goalkeeper deliberately removes his helmet and/or face mask in order to secure a stoppage of play, the Referee shall stop play as outlined above and assess the goalkeeper a minor penalty for delay of game. If the goalkeeper deliberately removes his helmet and/or face mask when the opposing team is on a breakaway, during the course of a penalty shot or shootout attempt, the Referee shall stop play and award a goal to the non-offending team.