IDK, thats a stretch to me. I think its just another glaring gap in the information they came up with to write the storyline. Even they say it only took out 60% of the US refining capability, why hit non allied sovereign nations if you left 60% of the US capacity intact?
The Mexicans/Division Cuba wouldn't bomb them, since they were in fact trying to annex Texas.
Unless your talking cruise missiles with conventional warheads, not sure if the Soviets even had them tbh, there are no other forces on American soil to hit them.
If, all those 130+ refineries were hit, all command centers and bomber wings/missile ranges, power plants (including nuclear) and major industrial centers I think we would be calling the game Gamma World not T2K.