We don't know that's what happened. I don't have any citations to support, but I do believe that teams can use emergency recalls when they have players who are game-time decisions provided that if the questionable player is able to play, and the team has enough healthy forwards (or defensemen or whatever), the player who was recalled on an emergency basis has to be scratched and sent down immediately (as an emergency condition no longer exists).
We know of one player who was a game-time decision for the game that O'Regan was called up for (although I cannot remember who). If there had been a second such player (and there would have had to be to create an emergency condition, as there was another forward healthy scratch for that game), then O'Regan could have been an emergency call-up, which would explain why he was a healthy scratch and was promptly sent back to Rochester: that's what would have had to have happened if he was an emergency call-up and the team ended up being able to ice twelve healthy forwards without him.
We do not know for certain that O'Regan was an emergency recall, as it requires information that we as fans do not have access to. However, the circumstantial evidence supports the idea. Barring some clarification from the media as to the nature of O'Regan's call-up, we do not have enough information to decide whether it was a mistake by management or not. I am inclined to believe, again due to the circumstantial evidence, that it was an emergency recall.