2A people presumably think that even though there are some edge cases that are bad (citizens owning nuclear weapons), the risk of unintended consequences of legislating against these cases is too great given the small-to-nil occurrence of these cases in the real world.
I’m sure people thought the same thing back when fully automatic weapons were first invented. “Sure gosh but who is ever going to tow around a Gatling gun with them?”
Nuclear weapons are an intentionally-absurd example to force a concession that there obviously must be limits. Frequently people will even acknowledge that the 2A probably doesn’t cover them (or at least oughtn’t be interpreted in a way that it does). From there it’s generally pretty easy to walk reasonable people backward towards MANPADs, and then further on down to things that aren’t quite so difficult to acquire.
Sure, but there’s a gap between “this would be bad” and “we need to change the law to stop it happening”, so what you think of as a concession isn’t necessarily conceding anything at all.