"But why is it ... bad... for birthers to question Obama's legitimacy to be president based on a document HE purposely did not release,"
You said the above. Not me.
As to your question, "Why is it bad?"
1. It doesn't matter that he didn't release the original--he released the document everyone in HI gets.
2. Where he was born (HI, hut, south pole) doesn't matter anyways.
3. It's bad because questioning his birth is some sort of bizarre conspiracy mania.
Stern was just playing the Fox and Friends morons questioning how the new certificate looks--"You see the green edges" etc. Kooks.