Sure, if you actually want to impale them, you can just stabilize them after the fact. But sometimes you don't want to do serious harm to them, or are too busy fighting to want to stop to treat a foe's wounds.
This is one of those rules which is mostly unrealistic, not especially intuitive, and absolutely a brain-saver around the table. It has a tendency to perhaps leave you slightly unsatisfied if you think about it a little too much, but it also does a perfect job of handling an fairly uncommon scenario in a simple way.