Can you prove that assault rifles have no other use? I ask because there's been PLENTY of people that use assault rifles and didn't hurt anyone else with them (shocking I know, but some people can own guns responsibly). There's people who think shooting at inanimate objects with heavy weaponry is fun so there's you other use right there.
Oh, well, lets risk the lives of other people so you can have some fun.
A nuke however is almost guaranteed to kill other people or destroy property that isn't yours if you use it, either by the blast or the fallout. I'd say that's good enough to make it an exception to the rule. This also makes it a national security risk although we still don't go to war with a country just because they have nukes.
And? Maybe I just want the security of knowing I can use it. I find it enjoyable and fun. That makes it alright, right?