The situation in Iran isn't anywhere near as black-n-white as you portray it to be. That kind of Good vs. Evil simplistic foreign policy thinking has gotten the US into quite a few problems - most recently with the catastrophic war in Iraq.

Not all the opposition protesters in the streets have been peaceful. They have attacked police stations, tried to lynch policemen, smashed shop windows, burned cars, etc. Any move to show support for the opposition, even if well intentioned and directed only at peaceful protesters, would be seen by many Iranians as meddling and as endorsement of violent mob actions to overthrow their government. We've done it before.

Of course it would be great if Iran, or any other country for that matter, became more open and free. But by foolishing "choosing sides", we would damn any Iranian group receiving that support with the stigma of a US/Western endorsement, and that would create a fierce backlash against them in Iran.

The leader of the so-called "reformers", Mousavi, is himself very much a creature of the establishment - he even had the blessing of the Supreme Leader and the Guardian Council going into the election. For example, an Iran under his leadership would be just as committed to nuclear proliferation as under Ahmadinajad. Any move to support the opposition would be seen as implicit - if not explicit - support for Mousavi. That would definitely be seen as meddling in the internal affairs of Iran.

An interesting point to ponder - both of our allies in Afghanistan and Iraq quickly congratulated Ahmadinejad on his reelection and they want to work constructively with the Iranian government. They have shown no appetite at all for this kind of interference in the internal affairs on Iran. Why? Being neighbors of Iran, perhaps they simply understand the situation a little more clearly. They don't want Iran messing with their countries (esp. Iraq) and they don't intend to mess with Iran. Let the Iranians work out their own issues.