It's true, who hasn't watched a hockey game and thought that the refs sucked. The difference though is that when watching a NHL game you can't honestly point to another league and think that the officiating is better. Of course there are bad calls & missed calls, but at the end of the day a NHL ref is one of the best refs in the world.
That can't be said for the MLS. I can point to 20 leagues with better officiating.