I would imagine it may be mostly because you can basically go all Hiroshima Nagasaki on the world once you get a hold of The Fat Man, Fallout 3's handheld nuclear catapult, or even the games premise revolving around a nuclear wasteland.  Yeah well kind of.  What Bethesda actually did was remove a side quest that involved detonating a nuclear bomb for an NPC by the name of Mr. Burke as well as naming The Fat Man, exactly that.  From what I hear Japanese gamers are mostly expressing disappointment with the edits.  I always wonder why they do these things for localization.  If it's obvious Japanese gamers want it in the game, put it in.  We shoot Nazi's to death, and soon, thanks to Call of Duty: World at War, while walking dead, so let the kids have their cake.  They obviously want it.