[Does so]

"...why not *expand* on the concept of MAD, to the rest of the world, to prevent other countries from using nuclear weapons...?"

Because a country that believes in "the 72 virgins that await its martyrs" can't be trusted?

"...This means to use neutron bomb nuclear weapons, that would kill all life in the country that used a nuclear weapon, though not destroying any of their buildings or contaminating their lands..."

There are a few Google references to Jimmy Carter's defunding our neutron weaponry, and I think there are no neutron weapons in our arsenal today.

[yefragetuwrabrumuy]

I believe you are correct. However, neutron bombs are not difficult to fabricate. Just add some extra shielding to reflect neutrons back in to the weapon, and set the bomb to an airburst detonation.

A double Cold War irony is that the Soviets preferred airburst weapons, that produce little fallout, whereas the US preferred ground burst weapons that produce much fallout. The Soviets also targeted US military targets; whereas the US targeted Soviet cities. On the surface it sounds like we were the bad guys, but there were sound strategic reasons to do so.