To quote Harold Ramis from Ghostbusters I.... "that would be incredibly dangerous..."
They can intercept in flight, so putting AAA, SAM, and any ABM capability into the region is defensive. Once the Russians have fired, then one way or other, a response is required.
Collectively, we cannot say that we are defensive if we commence an attack on legitimately Russian soil. Russians in Ukraine are invaders. Different beast.
So, the question to be asked is 'when does a defensive response become a pre emptive strike?' If we have proof positive that the Russians are about to start pressing big red buttons, is that not, in itself, the first action in a pre emptive strike by Russia and are we not therefore exercising our defensive philosophy by 'dealing with' (responding to) a known and imminent Russian pre emptive strike while the WMD are still in Russian territory? Why do we have to wait until thousands of innocents have been slaughtered in the west before we can start defending those that are still alive?
If we are being defensive by shooting down an inbound nuclear weapon in flight, what is the difference to taking it out as soon as it pops its nose above ground (or sea) level? What is the difference to taking it out while it is half way up the launch tube?