I think it was correct politically for the Administration to hold on their comments about Iran's after election mess. To me, the reason is simple -- the level of anti-americanism in that country is rather high. Speaking on behalf of the opposition would immediately crush their chances to get any political gains -- the ruling "group of people" would simply go on the radio/TV and say: "Look, those protesters are encouraged by America, funded by America and only serve American interests" -- remember who controls the media in Iran! The endorsement of opposition would have played against them. That already happened once when Bush was the president -- so they didn't want to repeat their earlier mistakes.
Of course, it didn't work anyways, so you can Monday-morning-quarterbacking it as much as you want....