My guess is that they are afraid that it would give a reason for all Western countries to support military actions against them. The interest of Iran is not to be seen as the source of instability and to leave US isiolated in their game to control ME oil.
See my comment above. Oil can not be targeted at the West.

Even a partial embargo would hurt Asian exporters and poor African countries before it hurt the US.

Given the US strategic reserve and domestic oil, the US could limp forward long after other countries, say South Korea (and I think China) had crashed and burned.