Why would it guarantee such an eventuality?

Strategic nukes - you know, the ones on ICBMs, the ones capable of such fallout scenario - are preventative. They’re MAD. Nobody wants to use them, everybody wants to have them. Their goal is total annihilation, maximum amount of radiation and destruction.

Tactical nuclear weapons are the ones you actually deploy - small, targeted charges with minimal radiation, very specific radius of destruction, which can be as small as a city block. Hell, the smallest nuclear weapon, the W54, has a fireball radius of as small as 4 meters, with a blast radius of 100-120m (severe damage), and ~300-400m (window breaking damage). In fact the largest issue with such a nuclear device is the relatively large gamma and neutron radiation radius - even though the fireball itself is small, the prompt radiation at detonation would vapourise everything in a 100m radius.

Basically if the US really wanted to play with nukes they could probably smuggle in maybe a dozen of these charges, and simultaneously vapourise the entire Iranian government as well as a handful of major landmarks in Tehran.

Or, you know, since the capital IS on the other side of a considerable mountain range, they can just drop a 2-3 megaton warhead on Tehran, wiping out a good 2/3 of the city (airburst), killing about 7 million people, without having ANY kind of radiation effect beyond a 50km radius.