Sudden reversals of policy like this are why the US is a dangerous foe, if one with a limited attention span or appetite for sacrifice, but an even more dangerous friend. We have a nasty habit of creating situations where we edge friends into complete dependence on us in a crisis and then kick the crutch out from under them with no detectable sense of guilt for the obvious consequences.
In addition to whole tragic goat-rope with the Shah, this also stinks of JFK's lethal conniving against the Diem regime in Viet Nam. That one turned out real good, dinnit?