It's becvause they haven't thought out the problem, just looking at it from a physical POV of 'what if some nutter exploded a nuke above the atmosphere.....?' they are trying to work out the problem (which can't be worked out), quite forgetting: why do it? how to do it (delivery etc)? what will be left on the ground after they have done it (because it will not be an act in isolation or without retaliation)? To give the problem some credibility, it was implied at the beginning 'it will almost certainly happen' (it won't).
So keep flogging that dead horse guys, there is no answer, but it's immensely amusing watching you try and get there. Work your slide rules frantically and leave the living to everyone else!