I think they don't update it because their policy is if you buy it once you don't have to pay for upgrades when they're released. Given that there are millions of legal users out there it would be a fairly taxing process on their resources to send every user a new key every time they updated their protection code.

Also it is my belief that you attract the interest of crackers if you keep changing your protection... it becomes a challenge to them. Once this happens you're pretty much wasting your time updating the protection. A case in point would be the so called EXE protectors (ASPack, Armadillo etc.), it is only a matter of (a very short) time after they are released before they are cracked