That's IF they saw it off the pegs. If they did, it would have been stopped. And how long was the net off the pegs before the goal? It could have been seconds before. Plus, if the game was stopped, that means the play would have been reset to a faceoff. Team Russia would not have gotten off that same shot from the same location after a reset.
Ok, it seems the net was knocked off well before the goal was scored. In this case, it was an error by the officials. I don't think they purposely let play continue, it was just something they missed. Officials miss calls in every game.