"There are currently two "unofficial" prizes the Nobel committee gives away: The Peace Prize and The Literature Prize. Both have never been officially sanctioned as part of the Alfred Nobel program."
You may want to check the Nobel Prize website to get accurate information--they include peace and literature with the other categories you mention, and they've been giving them away since 1901. Nothing unoffical about that. Also, remember that Carter received it in large part because of Habitat for Humanity as well as the work done by the Carter Center in attempting to peacefully resolve conflict.
He may have been a bad president, but it's hard to imagine begrudging or condemning him for his post-presidency work. Isn't Habitat for Humanity exactly the sort of non-federal-government, volunteer-based program that we are aiming for with faith-based initiatives?
Yeah, his post-presidential diplomacy really made a difference on that whole North Korean nuke thing, huh?