The purpose is to prevent the addition of a flash hider here in Connecticut. They are illegal on post ban firearms if the firearm has a pistol grip stock and a detachable magazine. Interestingly, muzzle brakes and suppressors are both legal - suppressors must have the appropriate BATFE stamp, of course.
I forgot there are other states with restrictions too haha. CT. is weird. Are you the guys that can have no more than 15 round magazines?