So, yes, again if you have nuclear capabilities you get the kid gloves treatment. If not, you can expect to be kicked around. Not exactly the sort of incentive structure you want to set up if eliminating nuclear states is your goal.
South Africa declared that their nuclear weapons program was to defend themselves from Soviet-backed regional threats. The USSR is neither the US nor the west, making the above statement incorrect.
Pakistan declared that their nuclear weapons program was a response to India's own program. India is neither the US nor the west, which would make it another counter-example.
The bottom line is that the behavior of states with nuclear weapons has, thus far, been categorically counter to the goal of reducing the proliferation of nuclear states.
> "We", of course, proclaim that this is nonsense.
The "this" referred to "the US/the west". It now refers to any threat from a nuclear neighbor.
The US, France, etc. did not regard South Africa's concerns about Soviet expansionist policies in southern Africa as nonsense. Moreover, South Africa had a three phase defense plan (quoting http://nuclearweaponarchive.org/Safrica/SABuildingBombs.html ):
Phase 1: Strategic uncertainty in which the nuclear deterrent capability
will not be acknowledged or denied.
Phase 2: Should South African territory be threatened, for example, by
the Warsaw Pact countries through the surrogate Cuban forces in
Angola, covert acknowledgement to certain international powers, e.g.
the USA, would be contemplated.
Phase 3: Should this partial disclosure of South Africa's capability not
bring about international intervention to remove the threat, public
acknowledgement or demonstration by an underground test of South Africa's
capability, would be considered.
Phase 2 would only work if the Western countries/USSR did not consider the threat as nonsense.Nor, I think, did people regard Pakistan's concern about an attack from nuclear-armed India to be "nonsense", though I haven't researched that issue.
Also, your modification is "...aggression by <fill-in-nuclear-state-here>." South Africa was not concerned with aggression from the USSR but by regional attacks from conventional forces from Angola, Mozambique, etc. with Soviet backing. I don't think this falls even under your modified version.