It is certainly not easy to reverse engineer these things. DOnt you think that was priority Number One when they designed it? Apply your logic to the Sky encryption system and there would be free TV for all by now, after all there are probably 10 million cards and digiboxes out there and have been for years.
Same goes for GSM encryption for example, even when the algorithms have been cracked it takes a huge amount of compute power to break in, which is why people go for easier methods like nicking the phone.
You could say the same about any system once the keys/passwords/details were released.