It is quite logical. Budapest Memorandum signed in 1994 where UK, USA and Russia guaranteed sovereignty and territorial integrity of Ukraine in exchange for nuclear weapons on Ukrainian soil which remained there after dissolution of USSR. If Russia wants to renege on Budapest Memorandum, Ukraine should get their nukes back.

Also you can see another side of the coin - Iran and DPRK would need to be insane to get rid of their nukes because only reason why big powers won't touch them is the nuclear poison pill, which Ukrainians gave up and look where they are.