Spinward Flow
SOC-14 5K
The ban is specifically for nuclear weapons, not for WMD.
Sounds like the prohibition is against "dirty weapons" that poison/pollute the biosphere for extended durations and require expensive cleanup efforts to environments. So "hazardous waste" nuclear effects (which just so happen to be rather energetic, which is what you want in weapons) are tightly controlled and banned.
So called "clean weapons" that rely on controlled nuclear reactions (such as fusion) to generate their effects, but with incredibly short half-lives (such as mesons, particle accelerators and other "energy weapon" style outputs) are permitted, if not outright encouraged, due to the limitations of what amounts to "second hand radiological effects" are going to be so limited that they do not contaminate battlefields after the fighting is finished (so no extensive cleanup required).
So as @McPerth says, the ban is on "indiscriminate 'dirty' nuclear weapons" that produce all kinds of secondary side effects ... as opposed to being a ban on weapons that "vaporize stuff more cleanly" in a mass destruction sort of way. Basically, low(er) tech disintegrators are fine (plasma, fusion, particle, meson, etc.) since they produce limited "collateral damage" and do not "despoil" the battlefield environment in a long(er) term way.
The difference between a "point and click" interface versus throwing a grenade and expecting someone else to come along and clean up after you. A difference between precision strikes and carpet bombing (so to speak) ... or if you prefer, the difference between a rapier and a garbage truck (both can kill you, but the rapier isn't quite as messy about it, or at least the mess is easier to clean up afterwards).