There's a lot more to this Petraeus email business, supposedly somebody close to the General was getting anonymous threatening emails. This person is important enough to get their problem handled by the FBI? Something pretty strange, especially that it forces Petraeus to resign.
Let's be clear, the notion that this somehow helps Obama is just nutz. Congress can still call him, Obama can still invoke executive privilege. This doesn't change anything, except that the Gen doesn't need to worry about his job, if he wanted to be chatty. Unless he could go to prison, that would make more sense with a resignation like this.