Well, I may agree with you that it is stupid to amend the constitution in such a way.
But in Arnold's defense, what would you have said in his position? "No, I don't think about it. Ever. I wouldn't want to be President if that amendment passed."?
Heck, if someone asked me that question I'd say the same thing. "Hey, Dude, you want to be President?"
I'd say, "Uh, sure."
My answer would have been simple: "You don't want me for President. I'd actually get things done and not give a damn who I piss off."