The main differences that the NHL has played up in explaining the lack of a suspension in this case is that it was a close game (so the resulting penalty mattered) and Cammalleri is a skilled player as Calgary’s top goal scorer this season. Cammalleri is not a goon. At 5’ 9" and 185 pounds he is on the small side for an NHL player. Thus Cammalleri is not a repeat offender and he was not "sending a message" at the end of a game.
Effectively these rulings make it OK for "skilled" players to do things that "goon" players cannot. It makes suspensions depend upon the person who commits the act instead of the act itself. This allows the NHL a coherent sounding reason to not suspend key players who will sell tickets and influence games, while suspending the fringe players.
In the playoffs, the NHL wants to send the message that suspensions will occur as usual (if needed) by suspending Carcillo, while also sending the message that they will not make unnecessary rulings to punish one team and hasten their playoff elimination, by not suspending Cammalleri. They cite the fact Carcillo is a repeat offender. They cite the fact that Carcillo was not playing in a key part of the game (which follows from him being more of a fringe player who does not play the most important minutes for his team). The NHL has setup a framework where they can cite a consistent set of principles and get away with only suspending only fringe players. The only problem is when a top level player becomes a repeat offender (as Chris Pronger has).
Suspending Carcillo and not Cammalleri for similar offences is the way the NHL usually operates. The more important a player is to the league and the more important the games the player will miss, the less likely he is suspended. Suspensions are for fringe players. These are the players who tend to become repeat offenders and tend to play in the points in the game where one might "send a message".