For the why the goblins would do that, I'd say because the GM felt it was dramatically appropriate. As for the damage move to be used sparingly, well, it's a hard move, you're expected to use it whenever there's a golden opportunity, not necessarily just rarely. Again, essentially, whenever the GM feels it's appropriate.
The GM in AW engine games isn't really scrutinized, in my experience, just do stuff and your good as long as your players don't think you're being cheap.