Sometimes, the work of the referees is harshly analyzed. The game is fast, a lot of things happen at the same time and these guys do their best to make the best calls possible. However, sometimes the rules get in the way of a fair outcome.
This one is not that clearly cut, but have a look at it.
The controversy comes out of the ruling. The puck was clearly inside the goal BEFORE the whistle was blown. As you probably know already, a referee can whistle to stop the play when he has lost the sight of the puck. In this case, the referee was going to whistle, but the puck appeared in the net and he whistled after. Jean-Gabriel Pageau was quick to point out the problem and it went into review.
As the call on the ice was a no-goal, it took a very clear and obvious review to reverse the outcome. In this case, the officials and the Toronto office decided it wasn't enough, and the call was maintained.
A lot of grumbling is heard about the review process in the league and it wouldn't be surprising to see some rules change in the near future.