Bad example. The Pandaren the Horde free arn't in chains, they were volunteers and it's implied they are worked too hard and intimidated to stay by the Alliance soldiers. On the Alliance side, the Pandaren didn't want to make a trade with the Horde, so they kidnapped their children and locked them up with magic. If anything, this proves the Horde are worse.
But it seems the moral equivilancy is considered 'canon' by Blizz, so I guess it doesn't matter what I see in game. As Brack said at the WoW announcement something like "Horde, you came to Azeroth in search of nothing more than a home". I guess it's mean old civilians of Stormwind who just don't respect the "searching" process of the Orcs that involves copious amounts of murder. Bunch of racists.