Beside the Constitutional problem, even if Obama was an Indonesian citizen at the time, he was still required by law to sign up for selective service:
"Almost all male U.S. citizens, and male aliens living in the U.S., who are 18 through 25, are required to register with Selective Service. It's important to know that even though he is registered, a man will not automatically be inducted into the military. In a crisis requiring a draft, men would be called in sequence determined by random lottery number and year of birth. Then, they would be examined for mental, physical and moral fitness by the military before being deferred or exempted from military service or inducted into the Armed Forces."
You are correct - even dual national citizens would have been required to register according to link you posted.
Then again, he was hanging around with the communist Frank Marshall Davis at the age of 18, someone who I seriously doubt would have ever encouraged him to register for the draft.
It now seems to make much more sense that birth certificate related reason for not registering for selective service would be more likely tied to being an foreign alien at the age of 18 or 19 in the year 1980.
So, if the draft form is in fact a true forgery, then, the reasons for him not registering could only be one of the three:
1. He refused to register under a form of personal protest
2. He was not a citizen of the US at the time
3. He simply forgot to register.