A cross check from behind isn't just a cross check. Taking a crosscheck unprepared to the back is 1000x worse then taking one while battling in front of the net. You try taking a kidney shot without warning, and then compare it to a cross check where you can brace yourself.
Honestly I'm amazed more people don't understand how dirty this play was. I don't like bringing hockey experience into it, but as someone who has played a fairly high level, and watched hockey for a long time, this play is dirty, and in no way is it "not that bad". I'm not saying Benn should get 5 games, but I do think he should get a game, the 5 minutes had no affect on the Edmonton game, so I believe he should serve a full game. The NHL can't just let people blind side players from the back with cross checks. It's a dirty play that has no place in the game.
Crosby does this routinely.