This article at CNN/Money has a bit more explanation; basically, the Wall Street view is that Northwest Airlines has to get its costs down, and the strike will actually help NWA do just that. By doing so, it will decrease the chances that the airline will have to file for bankruptcy.
I still don't see why the union is preparing itself for this course of action. No, their choices aren't very palatable, but how can they possibly think that they'll gain a better deal this way than by accepting NWA's offer? The results of a successful strike will be either forcing NWA into bankruptcy, or making themselves irrelevant and allowing NWA to hire replacement workers. That's what I don't get, myself.