explain why Obama go for the 'Executive Privilege'

Because he knows the GOP is fishing and would not stop at that. He saw what these unpatriotic assholes did to President Clinton.

How does an investigation of a real estate deal turn into 70 million dollars of taxpayer money spent on investigating a simple marital infidelity. Disgusting yes. But worth 70 million to investigate. hardly.

if it was just one rogue idiot

I don't think he or Holder are as up on the details as the reporter. She suggests that they should have done a better investigation. at the local level.