It specifically says it in the power that the only difference betwen it and a normal stun condition is that with the stun power, it could potentially last more ten one round, not that it can't be fixed with a HP. Think about it. What other power if you fail the save means that you are helpless and unable to perform any action, physical, Mental, or what ever for the duration, none of them. Its too powerful for a 2 point power for it not to have some weakness, Paralysis still allows mental actions, Stun does not allow anything so there has to be a balance, and that balance is the ability to use a HP to cancel it out. Remember, the book says nothing about not being able to use a HP to cancel it, it in fact states that the only difference between Stun Power and stunned condition is that Stunned condition can last for multiple turns, anything beyond that is simply inferred. I would point out that a character with the stun power can blow through an opponents HP very quickly and they are unable to use theose HPs for anything else on those turns.