Well in response to your first paragraph I was going to say the difference is HGH has negative side effects and can cause potentially severe long term health problems. But you addressed that right after, so I guess we can agree on that..
As far as pure effectiveness of the drug, I guess that really isn't the biggest issue. It's more the fact that it's dangerous and it's not ethical for everyone to have to risk their health in order to be on an even playing field.
Hypothetically if a 100% safe substance were developed that worked even better than HGH, it would bring up a lot of questions. How does the league want the game to be played? Shouldnt those who work hard in the gym be the ones reaping the physical benefits over someone who takes an injection? Does having stronger players significantly increase the risk of injury?