1. Just because nuclear, chemical, and biological weapons are lumped together doesn’t mean that they have comparable effects. “WMD” is a nearly worthless term, because nukes really aren’t like chem or bio weapons in scale of effect.
2. The US has massive conventional superiority over any foe or potential combination of foes. This, as they say, provides a deterrent.
3. The threat to use nuclear weapons in response to a chemical or biological attack wasn’t credible in the first place, in part because of the difference in scale, and in part because of the aforementioned massive US conventional superiority.
Of course, none of this will stop the stupid; it’s just useful to keep such things in mind.