My guess would be the old slippery slope argument. First, a guy with a P-51D with six fully functioning M2 .50 caliber BMGs gets to fly with them, then you'd have guys hanging Miniguns off either wing of their homebuilt ultralights, then some clown would mount a Vulcan 37mm rotary cannon on a Cessna, and so on. Not trying to argue, either, but a line has to be drawn somewhere before the "me, too" crowd starts thinking, " I can top that!".
If only we had a constitutional amendment that protected those sort of rights.....
I get what you are saying and for the most part agree. That being said, I would mortgage the house for P-51 Mustang hog hunts....