Yeah, 'cos what if the British come back, or the Red Injuns, or Martians, or .... All utter danglies, no civilian anywhere needs to own an assault rifle unless your entire population are army reserves - see Switzerland (virtually no gun crime).
Not saying it's rational, just saying that 1. it makes sense to the individual who's been brought up in a certain surrounding and 2. solving the problem isn't as simple as implementing laws, assuming you can even get those laws passed.