Border issues in the old days
Before the National Firearms Act, the makers of the Thompson promoted it as a way to protect ranches against border raiders.
This was practical: cross border raiding remains common to this day. If you want to get anything south of the border, all it costs is $10 mordida ("bite") to the crossing guards and you vehicle will not be searched. The trunk might be full of AKs or dead hookers or bomb-grade plutonium, it doesn't matter. For $10 no one will make messy inquiries.