The recent speculation over Iran's threats to shut down the Strait of Hormuz are probably a bluff. Shutting down the Strait might hurt the US but it would economically decimate the regime and invite a possible US military response.
Now other than the fact that the Obama administration's downright illegal threats of Iran are seen as legitimate, from flying a drone over Iranian airspace (now claimed to be a "pilot error") or shipping hundreds of bunker buster bombs to Diego Garcia to threaten Iran, there is a massive double standard in who gets to sanction who.
Keeping the sanctions in mind, Iran shutting down the Strait of Hormuz (on it's side) would in effect be a huge sanction on the US. However every commentator has said the US would "have to" stop the sanction by force.
So put simply, the US has a right to sanction Iran but Iran has no right to sanction the US back and in fact would effectively be an act of war.
Edit: Someone asked what makes Obama's threats illegal and it's a long held staple in International law that the threat of force is illegal:
4. All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state,