The reason that Obama is not going after Bush Cheney is the same reason no POTUS ever goes after the guy he replaced. They don't want to be in the crosshairs the moment they leave office. No POTUS ever has clean hands, certainly not Obama. If he were to go after Bush or Cheney you can bet in just a few years he'd be sharing a prison cell with them. That's just the way things work.
The world would have been different if he was President instead of the clown he had to lie for George W. Bush 2.
He is rare in the GOP party a man that can think for himself and not be brainwashed like so many in that political party.
Did in fact, as you claim, lie for George W. Bush 2 that does not say much for Powell's integrity.
Also Powell continuing to be a Republican does not annoy or bother in in any way.
You're Welcome
2016 = GOP All The Way
...and a Vietnam veteran who served 31 years in the Army for God and Country had this to say about Dick Cheney:
"Dick Cheney is, in a word, an idiot...Dick Cheney oughta be in jail for war crimes..."
http://www.thomhartmann.com/bigpicture/col-wilkerson-%E2%80%9Cdick-cheney-should-be-jail-war-crime