Where did you come up with that idea? The USA and Russia (USSR) have honoured every nuclear treaty they have ever signed going right back to the first nuclear treaty in 1963.
They've both gotten away with stuff in the past. But that's to be expected. Not really important as long as they still have even hundreds, rather than thousands of nukes--if there were ever to be an exchange, or even a one-sided perfect first strike with no counter-attack, the atmosphere would still be irradiated eventually--just look at the threat the Chernobyl cloud posed, and that wasn't even equivalent to a single fission-bomb.
Limiting their stockpiles to equal amounts is far more beneficial to the US simply because it has more advanced weapons systems than Russia does.
Even so, it's likely Russia is happy to dispense with nukes; of course there's no way of dispensing with the actual nuclear material but it's just a matter of putting it in some isolated underground storage facility and guarding it against thieves. Both their Cold-War era nuclear subs and missiles have been rotting away for some time and are becoming a potential hazard in addition to being a bitch to maintain.
The US is also willing to do this because nukes aren't really that useful when you can intimidate with a massive conventional force.
The main barrier to nuclear disarmament in general is US aggression. The invasion of Iraq and Afghanistan is making a lot of people nervous. The US will attack any state that goes rogue (i.e. seeks independence from the US system), that much is clear. The issue with states like Iran is that if they abandon their nuclear programs, it pretty much leaves them without a deterrent to US aggression down the road, as their conventional forces have no chance at defending their territory against such a massively superior force.
What slows down the present process is that Russia is hesitant because of the expansion of NATO into Eastern Europe and the establishment of missile bases there (under the guise of defence against Iran, as absurd a concept as that is).
