Technically it's not the torture causing the hp loss. If a girl loses the vast majority of her health after you choose to torture her, her rebelliousness was high enough that she attempted to escape and was injured when you and your goons subsequently thwarted her attempt. Torturing in and of itself only knocks off something like 3% of health each time.
That being said, it would be nice if there were some effect after a girl's escape attempt is thwarted. At the moment, she seems just as ready to mount a new attempt the next week and be severely injured once again. Perhaps a failed escape attempt could put a dent in rebellion, or have an elevated chance of removing Iron Will/Fearless traits.