That's the thing. The legality on it is... sketchy, and also different in every state.
If the gun itself were modified to support automatic mode, that's illegal.
If the gun simply has a trigger attachment that produces similar results of automatic.. That may not be illegal, because the gun itself isn't modified (I actually don't know the law regarding this)
But it is significantly easier to buy a legal gun, and mod it, than it is to buy a pre-modified or altered weapon. Thats where the narrative comes in.
Modified, illegal weapons, will circulate amongst bad groups no matter what the law is, they aren't confined by it (merely subject to economies). Legal weapons to be later modded can be affected by laws.