Importing Grapevine/Plant Material...(short/esoteric)

It's well known that it's against Federal regs to bring in your own grapevine material in your suitcase...though it appears to be done w/ some regularity. But I gather you can get into big trouble.
So...a question: The Cantina Zacagnini Montepulciano d'Abruzzo comes to this country with a cute little twig of a grapvine (I presume) tied around the neck w/ a string of straw. How do they get away against the prohibition of importing plant material??
Has anybody ever try to plant & grow that little twig??
Just curious.
Tom

I live 8 miles from the Canadian border. I can go to press houses in the Niagara Peninsula of Ontario and bring back up to $400 worth of crushed must and/or pressed juice in 23L pails as it is considered groceries. However, I cannot bring back fresh grapes, as they are an agricultural product.