Because he would lose a talking point that his critics are mainly part of a lunatic fringe? Because every minute he spends talking about those crazy birthers is a minute he doesn't have to explain how he is piling up a massive amount of broken campaign promises? Because it's a convenient distraction?
Umm no. If only 58% of Americans now believe Obama is a NBC, the negatives are far outpacing the benefits of using this as a smokescreen. The longer he goes without proving this, the lower that number goes. If Arizona passes the law and he refuses to campaign there, the rats will be jumping ship.