The issue is not the fallout but the fact that a nuclear weapon cannot be targeted - it by its very nature and design harms innocent civilians. Compare that to a fierarm, which can obviously be directed one way or the other. It's the same reason I wouldn't load my concealed carry handgun with FMJ rounds.
Realistically, if you fire a howitzer you have zero assurance that the shell will only land upon legitimate targets. The recent hostilities between Armenia and Azerbaijan are testament to the indiscriminate destructive power of military weapons.