I'm willing to bet this is defined by some base levels in their equations as a default utility. So they aren't trying to say what the optimal level for the various uses of guns are, they are using math to prove that people are being driven to purchase more guns than that level, whatever that level is.
This is why it’s sometimes hard to take the soft sciences seriously.
I have no idea why you be dismissive of using an unspecified variable as a baseline. People do it in every subject.