Thats not weasely, thats legal fact. Treaty's law. NATO has no claim over a single member nation's political acting. Not even if activating article 5!
The US has not signed that cluster ammo treaty, and thats it. It must not obey it, therefore. Collective decision making, you say? Regarding actions and policies authorized by NATO, yes. And even then the case you try to make is not as solid and clear as you maybe wish it would be.
The Ukraine as the victim has any right to defend itself in the way it sees fit. There is nothing to debate. It wants clusters, sees value in using them on its territory, and so be it.
However. Peace, bro.