"Suckers!"
Iran has to know that the US and the rest of the world will not enforce, or punish them when they fail to comply.
We already did that song and dance with Saddam Hussein for more than a decade, and even though it was absolutely certain that he was manipulating the UN inspectors, and pulling all sorts of stunts, the rest of the world, and most of the Democrats were aghast that George W. Bush actually tried to enforce the agreement.